tag:theconversation.com,2011:/global/topics/fluid-dynamics-438/articlesFluid dynamics – The Conversation2024-02-09T00:36:19Ztag:theconversation.com,2011:article/2228532024-02-09T00:36:19Z2024-02-09T00:36:19ZHigher, faster: what influences the aerodynamics of a football?<figure><img src="https://images.theconversation.com/files/573580/original/file-20240203-27-i63qjv.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C5472%2C3579&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">In addition to a player's ability to throw it, a number of factors will influence a ball's flight, including its size, inflation pressure and texture.</span> <span class="attribution"><span class="source">(Shutterstock)</span></span></figcaption></figure><p>With <a href="https://www.nfl.com/news/super-bowl-lvii-averages-audience-of-113-million-viewers-fox-sports">113 million viewers in the United States</a> and 40 million more around the world, the Super Bowl is the most popular sports event in North America. This year’s event on Sunday – with the added attraction of a <a href="https://www.cnn.com/videos/sports/2024/02/06/super-bowl-players-vegas-taylor-swift-wire-nc-vpx.cnn">romance in the spotlight</a> – promises to attract as many fans.</p>
<p>In Canada, the most recent Grey Cup final, last November, reached a <a href="https://twitter.com/RDS_RP/status/1726722586816430330">record audience</a> of 3.7 million viewers who tuned in to watch the Montréal Alouettes’ victory.</p>
<p>The two leagues definitely don’t enjoy the same popularity – far from it. Nor do they have the same rules. But there is another difference: although similar in appearance, the famous oval balls used in football have specific characteristics on both sides of the border that can affect their aerodynamics, i.e. the forces exerted by the air on the ball during its flight. The design and characteristics of the ball have an impact on the magnitude of these forces.</p>
<p>It might be news to football players, but their talent for throwing balls long distances is not the only thing that matters. A number of factors affect the ball’s aerodynamics, including the way it is made and its inflation pressure.</p>
<p>As a professor in the Department of Mechanical Engineering at Québec’s École de technologie supérieure, I am interested in experimental fluid dynamics. I study the physics of fluid flows and certain applications (e.g. propulsion of aquatic vehicles, aerodynamic applications). Fluid dynamics is a vast field and affects many aspects of our lives, such as the flow of blood in the heart, the flight of aircraft, the beautiful swirling patterns in Jupiter’s atmosphere or the perfect football pass for a touchdown.</p>
<h2>Ball size affects flight stability</h2>
<p>The NFL and CFL have the same <a href="https://cfldb.ca/faq/equipment/#:%7E:text=The%20CFL%20football%20dimensions%20are,to%2028%201%2F2%20inches">rules</a> regarding the dimensions of their balls. They must be between 11" and 11.25" long. They must also be inflated to between 12.5 psi and 13.5 psi, giving them a maximum circumference of between 28" and 28.5" around the length and between 21" and 21.25" around the width.</p>
<p>These dimensions are important. The football acts like a gyroscope. The higher the speed of rotation, the more stable the ball will be during its flight. Different dimensions can therefore have specific effects on the stability of the ball’s flight.</p>
<figure class="align-center ">
<img alt="An American football player catches a ball in mid-flight on a field" src="https://images.theconversation.com/files/573219/original/file-20240203-25-y5at9n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/573219/original/file-20240203-25-y5at9n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=438&fit=crop&dpr=1 600w, https://images.theconversation.com/files/573219/original/file-20240203-25-y5at9n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=438&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/573219/original/file-20240203-25-y5at9n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=438&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/573219/original/file-20240203-25-y5at9n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=551&fit=crop&dpr=1 754w, https://images.theconversation.com/files/573219/original/file-20240203-25-y5at9n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=551&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/573219/original/file-20240203-25-y5at9n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=551&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The size of the football matters. The ball acts like a gyroscope. The higher the speed of rotation, the more stable the ball will be during its flight.</span>
<span class="attribution"><span class="source">(Shutterstock)</span></span>
</figcaption>
</figure>
<p>A larger circumference suggests that more of the ball’s mass is located away from its centre line. This means that it will have a higher moment of inertia (resistance to rotation) and, therefore, that the same force applied to make it rotate will result in a lower speed of rotation.</p>
<h2>Two stripes and laces make a difference</h2>
<p>While there are two white stripes on the Canadian ball, as well as laces, American rules don’t mention these.</p>
<p>The differences between the Canadian and American balls can have an effect on their drag. A drag force is the resistance to a moving object in a fluid. In this case, it is mainly the resistance caused by the air (a fluid), which is called form or pressure drag.</p>
<p>Let’s take the example of a golf ball. Its dimples encourage turbulence, which allows the airflow to stick to the ball and reduce its total drag. Less drag means the ball can fly further with the same force applied.</p>
<p>The laces on a football and any other significant modification to its surface (a logo, a valve), in combination with the rotation of the ball, will to some extent have the same effect. It would be interesting to study how <a href="https://www.engineering.com/story/the-aerodynamics-of-a-football">these differences</a> between NFL and CFL footballs affect their respective drag.</p>
<h2>NFL or CFL, which ball is better?</h2>
<p>To do this, we could use a wind tunnel (an experimental installation in the form of a tunnel with a controlled airflow) to simulate the movement of air (fluid flow) around the two balls that will be fixed in space, put into rotation and subject to an airflow speed that would imitate the balls’ speed of flight.</p>
<p>An aerodynamic force balance could be used to measure the differences in drag between the two balls subjected to the same conditions. Ideally, to eliminate other factors of variability, the two balls would have the same dimensions.</p>
<p>The passage of air around the ball could be visualized using smoke or particle image/tracking velocimetry. The latter is a method in which the air is seeded with particles (helium-filled soap bubbles or oil droplets). The movement of these particles could then be captured using a camera to quantify the airspeed at all points around the ball. This would allow regions of flow separation and recirculation to be seen, and provide an idea of the distribution of aerodynamic forces around the ball.</p>
<figure class="align-center ">
<img alt="A gloved hand holds a football on a grassy surface" src="https://images.theconversation.com/files/573221/original/file-20240203-21-3s2qf1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/573221/original/file-20240203-21-3s2qf1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/573221/original/file-20240203-21-3s2qf1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/573221/original/file-20240203-21-3s2qf1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/573221/original/file-20240203-21-3s2qf1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/573221/original/file-20240203-21-3s2qf1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/573221/original/file-20240203-21-3s2qf1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A ball about to be kicked. A number of factors will influence the aerodynamics of the ball.</span>
<span class="attribution"><span class="source">(Shutterstock)</span></span>
</figcaption>
</figure>
<p>Different rotation speeds and flight speeds could be examined, as there is always the possibility of developing flow instabilities, which would lead to a change in its behaviour around the ball. </p>
<p>This would help determine whether the NFL or CFL ball is better.</p>
<h2>Ball texture influences drag</h2>
<p>There is another type of drag, this one attributable to the friction between the air and the surface of the ball. This is called friction drag.</p>
<p>It depends mainly on the texture of the ball and its speed. The rougher the texture of the ball, the greater the friction drag for the same speed. Similarly, a faster ball speed will have a higher friction drag.</p>
<p>By reducing the form drag, we further reduce the total drag of the ball, which can therefore go further and faster on the football field.</p>
<h2>And then there’s the weather!</h2>
<p>The weather also plays a role in the aerodynamics of the football.</p>
<p>Cold or hot temperatures can affect the size of the ball by reducing or increasing the air pressure inside it.</p>
<p>Similarly, temperature can have some effect on the material properties of the ball, with colder temperatures making it stiffer and warmer temperatures making it softer.</p>
<p>Temperature and humidity also play a role in the physical properties of air, altering its density and viscosity.</p>
<p>Rain will also directly affect drag as, in a sense, it affects the texture of the ball’s surface as felt by the air.</p>
<p>But that won’t be an issue in Las Vegas on Feb. 11 for the Super Bowl game, since Allegiant Stadium is covered.</p><img src="https://counter.theconversation.com/content/222853/count.gif" alt="La Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Giuseppe Di Labbio ne travaille pas, ne conseille pas, ne possède pas de parts, ne reçoit pas de fonds d'une organisation qui pourrait tirer profit de cet article, et n'a déclaré aucune autre affiliation que son organisme de recherche.</span></em></p>A football’s dimensions, pressure and texture affect its aerodynamics, i.e. the forces exerted by the air on the ball as it flies.Giuseppe Di Labbio, Professeur adjoint, École de technologie supérieure (ÉTS)Licensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2212842024-01-22T20:05:54Z2024-01-22T20:05:54ZTiny water-walking bugs provide scientists with insights on how microplastics are pushed underwater<figure><img src="https://images.theconversation.com/files/570372/original/file-20240119-29-p4evyd.jpg?ixlib=rb-1.1.0&rect=242%2C9%2C5985%2C4146&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">You may hardly feel a raindrop, but for some tiny insects, one drop can have an intense impact. </span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/raindrop-royalty-free-image/682204834?phrase=raindrop+falling&adppopup=true">Mendowong Photography/Moment via Getty Images</a></span></figcaption></figure><p><a href="https://doi.org/10.1016/j.envpol.2016.06.074">Microplastics are tiny plastic particles</a> that can cause <a href="https://doi.org/10.3390/su151410821">big problems</a> when they enter the water supply. One way my <a href="https://www.dickersonlab.com/">fluid dynamics lab</a> explores microplastic movement is by studying how tiny water-walking insects are pushed underwater by raindrops.</p>
<p>Exposure to microplastic pollution can pose health risks, such as <a href="https://doi.org/10.3390/nano11020496">respiratory and digestive problems</a>, increased <a href="https://doi.org/10.5334/aogh.4056">risk of diabetes</a> and <a href="https://doi.org/10.3390/ijerph17041212">disrupted sleep</a>. But <a href="https://mabe.utk.edu/people/andrew-dickerson/">physicists like me</a> can study how they move through water to learn how to clean them up. </p>
<p><a href="https://www.britannica.com/animal/water-strider">Water striders</a> are tiny insects that can <a href="https://doi.org/10.1038/nature01793">walk on water</a>. They’re abundant in humid, rainy areas, and some species go their entire lives without ever touching land. Raindrops can weigh more than 40 times a water strider, and during storms they occasionally strike striders directly. The drops form a tiny crater under the surface of the water that envelops the strider before jettisoning it out as the crater collapses back to the surface. </p>
<p>The water striders have strong exoskeletons that allow them to survive being hit by a raindrop. Because these insects are <a href="https://doi.org/10.1016/j.matpr.2022.04.901">water-repellent</a> and very lightweight, they usually bounce right back. But sometimes the raindrops will form a second, smaller crater right below the surface. The second crater usually forms from a large, fast drop.</p>
<p>If the water strider finds itself inside this second crater, it could get trapped under the water. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/570376/original/file-20240119-18-v7fklk.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Four photos showing a raindrop colliding with the surface of the water, the first showing a dip below the surface in which a small, long-legged insect floats, the second showing the insect meeting the surface, and the third showing another small sip with the insect inside, and the fourth showing the insect submerged under the water." src="https://images.theconversation.com/files/570376/original/file-20240119-18-v7fklk.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/570376/original/file-20240119-18-v7fklk.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=606&fit=crop&dpr=1 600w, https://images.theconversation.com/files/570376/original/file-20240119-18-v7fklk.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=606&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/570376/original/file-20240119-18-v7fklk.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=606&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/570376/original/file-20240119-18-v7fklk.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=761&fit=crop&dpr=1 754w, https://images.theconversation.com/files/570376/original/file-20240119-18-v7fklk.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=761&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/570376/original/file-20240119-18-v7fklk.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=761&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Raindrops form two craters, the second of which can submerge striders.</span>
<span class="attribution"><span class="source">Daren A. Watson and Andrew K. Dickerson, from the Proceedings of the National Academy of Sciences</span></span>
</figcaption>
</figure>
<p>In my lab’s <a href="https://doi.org/10.1073/pnas.2315667121">latest study</a>, we captured water striders from local ponds and released falling drops above their tanks. We used high-speed videography and image analysis to see how fast the insects submerged when the raindrops hit them.</p>
<p>My colleagues and I also measured the acceleration of the second, smaller crater. This crater retracts quickly – according <a href="https://doi.org/10.1073/pnas.2315667121">to our measurements</a>, 50 times the acceleration due to gravity. Water striders cannot support themselves inside this second bubble, as the surface they’re on moves upward so quickly, and they might fall underwater and become submerged. If that happens, the water striders make powerful swimming strokes to try to resurface.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/570392/original/file-20240119-25-h4qkfd.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two illustrations show the process of a strider underwater using its legs to kick up to the water's surface." src="https://images.theconversation.com/files/570392/original/file-20240119-25-h4qkfd.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/570392/original/file-20240119-25-h4qkfd.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=121&fit=crop&dpr=1 600w, https://images.theconversation.com/files/570392/original/file-20240119-25-h4qkfd.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=121&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/570392/original/file-20240119-25-h4qkfd.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=121&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/570392/original/file-20240119-25-h4qkfd.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=153&fit=crop&dpr=1 754w, https://images.theconversation.com/files/570392/original/file-20240119-25-h4qkfd.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=153&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/570392/original/file-20240119-25-h4qkfd.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=153&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Striders can often kick back up to the surface if they get submerged, unlike plastic particles.</span>
<span class="attribution"><span class="source">Daren A. Watson and Andrew K. Dickerson, from the Proceedings of the National Academy of Sciences</span></span>
</figcaption>
</figure>
<h2>Why it matters</h2>
<p>Like water striders, microplastics are very light and often water-repellent. They tend to move on top of the water in a similar way, and raindrops can submerge them. When pollutants get submerged, they’re <a href="https://doi.org/10.1007/s10311-020-00983-1">harder to clean up</a>, and <a href="https://journals.openedition.org/factsreports/5257">marine life might consume</a> them.</p>
<p>Our research tells us that the second crater’s quick acceleration toward the water’s surface plays a big part in sinking tiny particles – water striders and microplastics alike.</p>
<p>Studying how small particles and organisms disperse in water could help scientists figure out how to prevent and mediate microplastic pollution. </p>
<h2>What still isn’t known</h2>
<p>Water striders are so water-repellent that they carry a bubble around them <a href="https://doi.org/10.1017/S0022112008002048">called a plastron</a> when pushed underwater.</p>
<p>In the lab, the more times they are struck by drops before repelling away the water, the more likely water striders are to remain submerged <a href="https://doi.org/10.1073/pnas.2315667121">for extended periods</a>.</p>
<p>Raindrop impacts seem to deplete the plastron. We don’t yet know how many repeated impacts striders can tolerate and how chemical pollutants in waterways affect their resistance to submersion. </p>
<h2>What’s next</h2>
<p>Future work will replace the water striders in our experiments with floating particles that mimic microplastics, with a range of size, density and water-repellency. We expect larger particles to make the drops break apart upon contact, while the smaller particles will likely get carried into the air, or <a href="https://doi.org/10.1186/s43591-021-00018-8">aerosolized</a>, by the splash.</p>
<p>And the striders aren’t just good models for microplastic movement. Studying water striders’ legs as they swim could also help researchers design underwater robots.</p><img src="https://counter.theconversation.com/content/221284/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Dickerson receives funding from the National Science Foundation. </span></em></p>Microplastic pollution is a growing problem − one lab is looking at tiny insects as inspiration for how these pollutants might move through water.Andrew Dickerson, Assistant Professor of Mechanical, Aerospace and Biomedical Engineering, University of TennesseeLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2197852024-01-11T13:24:53Z2024-01-11T13:24:53ZOtters, beavers and other semiaquatic mammals keep clean underwater, thanks to their flexible fur<figure><img src="https://images.theconversation.com/files/566392/original/file-20231218-18-2f1ege.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2136%2C1467&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Otters and other semiaquatic mammals can keep clean even in dirty water. </span> <span class="attribution"><a class="source" href="https://newsroom.ap.org/detail/ANIMALSINTROUBLE/88740e31a4f1471ea8048eda247fbceb/photo?Query=otter&mediaType=photo&sortBy=&dateRange=Anytime&totalCount=191&digitizationType=Digitized&currentItemNo=13&vs=true&vs=true">AP Photo/Kirsty Wigglesworth</a></span></figcaption></figure><p>Underwater surfaces can get grimy as they accumulate dirt, algae and bacteria, a process <a href="https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/biofouling">scientists call “fouling</a>.” But furry mammals like beavers and otters that spend most of their lives wet manage to avoid getting their fur slimy. These anti-fouling abilities come, in part, from one of fur’s unique properties — that each hair can bend and flex as an animal moves.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/_ev_ukj9HCU?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Fouling on boats and machinery can be a big problem, and scientists are searching for ways to prevent it.</span></figcaption>
</figure>
<p><a href="https://www.dickersonlab.com/">I’m a mechanical engineer</a> who studies fluid dynamics, or how liquids behave. My team recently <a href="https://doi.org/10.1098/rsif.2023.0485">published a study</a> showing that fur that was allowed to move back and forth in a flow of dirty water accumulated less than half the amount of dirt as fur that was <a href="https://doi.org/10.1098/rsif.2021.0904">held stiff from both ends</a>.</p>
<p>While lots of animals have <a href="https://theconversation.com/body-hair-helps-animals-stay-clean-and-could-inspire-self-cleaning-technologies-50445">fur that seems to clean itself</a>, semiaquatic mammals have the most grime-resistant, or “anti-fouling,” fur.</p>
<p>Our recent study compared fur fibers from beavers, otters, springbok, coyotes and more using a flow of water containing <a href="https://pubchem.ncbi.nlm.nih.gov/compound/Titanium-Dioxide">titanium dioxide</a>, a common additive in cosmetics. Titanium dioxide readily attaches to surfaces like skin. Our team pumped the dirty water over individual fibers in a closed loop for 24 hours, then cleaned the fibers to measure how much titanium dioxide they’d accumulated.</p>
<p>My colleagues and I then used mathematical techniques to combine all of fur’s properties into a single number that predicts its anti-fouling behavior. We considered each fur strand’s ability to bend, how fluid flows over it and other unique features of each species. </p>
<p><a href="https://doi.org/10.1098/rsif.2023.0485">We found</a> that the ability to flex was critical for keeping the animal’s fur clean. </p>
<h2>Why it matters</h2>
<p>Fouling can <a href="https://doi.org/10.1016/j.cis.2020.102336">damage the affected surface</a>. When fur fouls, the arrangement of individual strands across the animal’s pelt is disrupted, and the animal might struggle to stay warm or dry.</p>
<p><a href="http://dx.doi.org/10.3390/polym13060846">Industrial repellent methods</a> used to protect the bottom of ships and the insides of pipes often <a href="http://dx.doi.org/10.1016/j.marpolbul.2004.02.034">employ harmful chemicals</a> and consume energy and materials, unlike naturally evolved solutions.</p>
<p>Figuring out how fur stays clean naturally could lead to more environmentally friendly solutions for repelling fouling in the water supply, in marine environments and even in the medical field. Solutions could include surfaces with parts that can flex and move or that have little hairs on a surface. </p>
<p>Research into fur also reveals more about how these mammals have evolved to survive across a variety of environments.</p>
<h2>What still isn’t known</h2>
<p>Animal fur and the fouling process <a href="https://doi.org/10.1016/j.cis.2020.102336">are both complex</a>, so we still don’t fully understand how all the intricate properties of fur, from texture and length to cross-sectional shape and environmental conditions, contribute to cleanliness. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/566393/original/file-20231218-23-v903xy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A close-up of an otter's coat, with lots of brown fur packed closely together." src="https://images.theconversation.com/files/566393/original/file-20231218-23-v903xy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/566393/original/file-20231218-23-v903xy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/566393/original/file-20231218-23-v903xy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/566393/original/file-20231218-23-v903xy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/566393/original/file-20231218-23-v903xy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/566393/original/file-20231218-23-v903xy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/566393/original/file-20231218-23-v903xy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Since hairs in fur are packed densely, they brush against each other and don’t always move individually.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/otter-fur-close-up-royalty-free-image/691551942?phrase=fur%2Bup%2Bclose%2Botter">Hailshadow/iStock via Getty Images Plus</a></span>
</figcaption>
</figure>
<p>The strands of hair in fur don’t always move individually. On an animal, the hairs are packed tightly, and they likely clean each other by rubbing as their host moves. We can’t yet tell if rubbing and moving affect the host animal’s cleanliness. </p>
<h2>What’s next</h2>
<p>My colleagues and I have just scratched the surface of the mystery of furry mammal cleanliness, and there’s plenty more we can test. Future work could expose fur to biological foulers like bacteria and algae, or look at the role patches of fur play in cleanliness. </p>
<p>The only known mammal that does succumb to fouling is the sloth – <a href="https://news.mongabay.com/2014/03/sloths-moths-and-algae-a-surprising-partnership-sheds-light-on-a-mystery/">algae grows on their fur</a>. </p>
<p><em>The <a href="https://theconversation.com/us/topics/research-brief-83231">Research Brief</a> is a short take on interesting academic work.</em></p><img src="https://counter.theconversation.com/content/219785/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Dickerson receives funding from the National Science Foundation. </span></em></p>The bottoms of boats and docks can accumulate lots of dirt, but semiaquatic animals like otters avoid having ‘fouled’ fur. Their secret could one day help keep underwater infrastructure clean.Andrew Dickerson, Assistant Professor of Mechanical, Aerospace and Biomedical Engineering, University of TennesseeLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2087612023-07-10T20:11:22Z2023-07-10T20:11:22ZDoes the direction water rotates down the drain depend on which hemisphere you’re in? Debunking the Coriolis effect in your sink<figure><img src="https://images.theconversation.com/files/534774/original/file-20230629-15-plnypv.jpg?ixlib=rb-1.1.0&rect=581%2C1032%2C4218%2C2604&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The idea that the Coriolis force influences how water drains frequently appears in popular culture and urban legends.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/es/image-photo/jet-water-flows-into-sink-concept-1399159301">frantic00 / Shutterstock</a></span></figcaption></figure><p>In countries near the Earth’s equator, tourists are often dazzled by a demonstration of a mysterious physical phenomenon. A presenter will position three buckets of water – one in the Northern Hemisphere, one in the Southern Hemisphere, and one directly on the equator – and let the water drain out.</p>
<p>Tourists are shown that, as the water drains, the water in the northern bucket rotates in one direction, the water in the southern bucket rotates in the other direction, and the water at the equator doesn’t rotate at all.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Pb69HENUZs8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Tourists in countries near the equator, like Uganda and Ecuador, are amazed by attractions that claim to demonstrate the Coriolis effect.</span></figcaption>
</figure>
<p>The demonstrator might claim that this strange phenomenon is governed by physics, that it’s an example of the <a href="https://education.nationalgeographic.org/resource/coriolis-effect/">Coriolis effect</a>.</p>
<p>The intriguing nature of the Coriolis effect has led to its frequent appearances in urban legends and popular culture, from <a href="http://www.lghs.net/ourpages/users/dburns/ScienceOnSimpsons/Clips_files/Coriolis.m4v">TV shows</a> to <a href="http://www.gamefaqs.com/boards/939217-call-of-duty-4-modern-warfare/43834255/480093367">video games</a>.</p>
<p>The Coriolis effect is based on the idea that the spinning of the Earth introduces a physical force, known as the Coriolis force, which affects the way objects appear to move to us Earthbound observers. The Coriolis force causes objects on the Earth’s surface to be deflected in different directions depending on whether they are above or below the equator. The effect is strongest near the poles and weakest at the equator.</p>
<p>The Coriolis effect is legitimately responsible for the behavior of some natural phenomena, like hurricanes, that meterologists and physical oceanographers like <a href="https://scholar.google.com/citations?user=YdRRHIQAAAAJ">the two</a> <a href="https://0-scholar-google-com.brum.beds.ac.uk/citations?user=cQOa614AAAAJ&hl=fr">of us</a> study. But in domestic settings, the spinning of the Earth actually has very little effect on how water behaves. Math can explain how this works – or doesn’t work – in a kitchen sink.</p>
<h2>The math behind the phenomenon</h2>
<p>Geophysicists use certain mathematical equations, known as the <a href="https://www.britannica.com/science/Navier-Stokes-equation">Navier-Stokes equations</a>, to describe the behavior of fluids. Roughly, the Navier-Stokes equations relate the change of fluid velocity – how the fluid moves – to the forces acting on the fluid, subject to a few physical constraints. For example, the equations assume that the overall amount of fluid in the system doesn’t change over time.</p>
<p>But just because physicists and mathematicians can write down these equations, it doesn’t mean we can solve them. In fact, these equations are so difficult to solve that you would <a href="https://theconversation.com/millennium-prize-the-navier-stokes-existence-and-uniqueness-problem-4244">win a Millennium Prize and US$1 million</a> if you could do it.</p>
<p>Although there is no known complete solution to Navier-Stokes equations, meteorologists and physical oceanographers can still obtain useful partial solutions. One way to obtain these partial solutions is to compare various terms in the Navier-Stokes equations to determine which ones are most important. </p>
<p>These comparisons are often recorded as ratios and have no associated physical unit, thereby earning them the name “<a href="https://www.sciencedirect.com/topics/chemistry/dimensionless-number">dimensionless numbers</a>.”</p>
<h2>What happens in your sink?</h2>
<p>In the context of the Coriolis effect, perhaps the most important dimensionless number is the <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/rossby-number">Rossby number</a>, named for the early 20th-century meteorologist <a href="https://www.britannica.com/biography/Carl-Gustaf-Arvid-Rossby">Carl-Gustav Rossby</a>. The Rossby number compares the dynamics of the fluid with the Earth’s rotation rate, taking into account how big the system is and how fast it’s moving. </p>
<p>A small Rossby number indicates that the Coriolis force has a strong effect on the system, while a large Rossby number signifies that the Coriolis force has a negligible effect. For example, the Rossby number for an average hurricane is of the order of 1, indicating that the dynamics of the fluid and the Earth’s rotation rate are of similar relevance. It is <a href="https://education.nationalgeographic.org/resource/coriolis-effect-1/">true that hurricanes</a> tend to rotate clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere.</p>
<p>The same math that applies to large-scale phenomena like hurricanes also applies to the water in your bathroom sink. In this setting, the system is relatively small, and so the Rossby number will be much larger than 1 – more than 10,000 times larger. This observation indicates that the Coriolis force is negligible on water draining in a bathroom sink.</p>
<p>In fact, the Rossby number predicts that the water would need to move at an almost imperceptible speed for the Coriolis force to become significant. So even though the way water swirls down the drain may be consistent, that isn’t due to the Coriolis effect.</p>
<h2>So what did the tourists see?</h2>
<p>The same logic applies to the equatorial attractions. Given the size of the system, physical oceanographers can comfortably conclude that the Coriolis force is not responsible for what the tourists see in those buckets or bowls. </p>
<p>This conclusion is also supported by examining the same kind of presentation in different countries. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/z9JRL0YIu18?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Different videos of the Coriolis effect show the water spinning in different directions in the same hemisphere.</span></figcaption>
</figure>
<p>The water in the Northern Hemisphere rotates counterclockwise in one video but clockwise in another video. If the rotation were due to the Coriolis effect, the result would be the same in both videos. </p>
<p>Although physical oceanographers can’t deny what the tourists see, we know that the magic trick isn’t due to the Coriolis effect at such a small scale.</p><img src="https://counter.theconversation.com/content/208761/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Las personas firmantes no son asalariadas, ni consultoras, ni poseen acciones, ni reciben financiación de ninguna compañía u organización que pueda obtener beneficio de este artículo, y han declarado carecer de vínculos relevantes más allá del cargo académico citado anteriormente.</span></em></p>This physical effect does explain how some massive natural phenomena like hurricanes behave. But on the scale of water in your sink – not so much.Francisco José Machín Jiménez, Profesor Titular de Universidad. Oceanógrafo Físico, Universidad de Las Palmas de Gran CanariaBorja Aguiar González, Personal Docente e Investigador, Universidad de Las Palmas de Gran CanariaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1957922023-01-09T13:18:31Z2023-01-09T13:18:31ZHow cancer cells move and metastasize is influenced by the fluids surrounding them – understanding how tumors migrate can help stop their spread<figure><img src="https://images.theconversation.com/files/502978/original/file-20230103-70338-2503wk.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C2476%2C1209&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Tumor cells traverse many different types of fluids as they travel through the body.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/spreading-cancer-cell-illustration-royalty-free-illustration/1407269122">Christoph Burgstedt/Science Photo Library via Getty Images</a></span></figcaption></figure><p><a href="https://doi.org/10.1016/C2020-0-03305-0">Cell migration</a>, or how cells move in the body, is essential to both normal body function and disease progression. Cell movement is what allows body parts to grow in the right place during early development, wounds to heal and tumors to become metastatic.</p>
<p>Over the last century, how researchers understood cell migration was limited to the effects of biochemical signals, or <a href="https://bio.libretexts.org/Bookshelves/Microbiology/Book%3A_Microbiology_(Boundless)/7%3A_Microbial_Genetics/7.21%3A_Sensing_and_Signal_Transduction/7.21A%3A__Chemotaxis">chemotaxis</a>, that direct a cell to move from one place to another. For example, a type of immune cell called a neutrophil migrates toward areas in the body that have a <a href="https://doi.org/10.4049/jimmunol.155.3.1428">higher concentration of a protein called IL-8</a>, which increases during infection.</p>
<p>In the past two or three decades, however, scientists have started to recognize the importance of the <a href="https://www.mechanobio.info/">mechanical, or physical, factors</a> that play a role in cell migration. For example, human mammary epithelial cells – the cells lining the milk ducts in the breast – <a href="https://doi.org/10.1126/science.aaf7119">migrate toward areas of increasing stiffness</a> when placed on a surface with a stiffness gradient.</p>
<p>And now, instead of focusing on just the effect of the “solid” environment of cells, researchers are turning toward their “fluid” environment. As a <a href="https://scholar.google.com/citations?user=nKmJNpQAAAAJ&hl=en">theoretician</a> trained in applied mathematics, I use mathematical models to understand the physics behind cell biology. My colleagues <a href="https://scholar.google.com/citations?user=otbcd-EAAAAJ&hl=en">Sean X. Sun</a> and <a href="https://scholar.google.com/citations?user=sMrPz8sAAAAJ&hl=en">Konstantinos Konstantopoulos</a> and I were among the pioneering scientists who discovered how <a href="https://doi.org/10.1242/jcs.240341">water and hydraulic pressure</a> influence cell migration through theoretical models and lab experiments. In our recently published research, we found that human breast cancer cell migration is enhanced by the <a href="https://doi.org/10.1038/s41467-022-33683-1">flow</a> and <a href="https://doi.org/10.1038/s41586-022-05394-6">viscosity</a> of the fluids surrounding them, clarifying one of the factors influencing how tumors metastasize.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/FD-A0MhYc7Y?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Cells can move in different ways.</span></figcaption>
</figure>
<h2>How fluids affect cell migration</h2>
<p>Cells in the human body are constantly exposed to fluids of <a href="https://doi.org/10.1038/s41586-022-05394-6">different physical properties</a>. Water is one such fluid that can direct cell migration. For example, we found that <a href="https://doi.org/10.1038/s41467-022-33683-1">how water flows across the membranes</a> of breast cancer cells influences how they move and metastasize. This is because the amount of water traveling in and out of a cell causes it to shrink or swell, inducing movement by translocating different parts of the cell.</p>
<p>The viscosity, or thickness, of body fluids varies from organ to organ, and from health to disease, and this can also affect cell migration. For example, the fluid between cancer cells in tumors is more viscous than the fluid between normal cells in healthy tissues. When we compared how quickly breast cancer cells move in confined channels filled with fluid of normal viscosity versus fluid of high viscosity, we found that cells in high viscosity channels <a href="https://doi.org/10.1038/s41586-022-05394-6">counterintuitively sped up</a> by a significant 40%. This discovery was unexpected because the fundamental laws of physics tell us that inert particles should slow down in high viscosity fluids due to increased resistance.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/502975/original/file-20230103-105030-c8xq8d.gif?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Animation comparing two fluids with lower and higher viscosity." src="https://images.theconversation.com/files/502975/original/file-20230103-105030-c8xq8d.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/502975/original/file-20230103-105030-c8xq8d.gif?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=463&fit=crop&dpr=1 600w, https://images.theconversation.com/files/502975/original/file-20230103-105030-c8xq8d.gif?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=463&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/502975/original/file-20230103-105030-c8xq8d.gif?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=463&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/502975/original/file-20230103-105030-c8xq8d.gif?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=582&fit=crop&dpr=1 754w, https://images.theconversation.com/files/502975/original/file-20230103-105030-c8xq8d.gif?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=582&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/502975/original/file-20230103-105030-c8xq8d.gif?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=582&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The blue fluid on the left has a lower viscosity relative to the orange fluid on the right.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/File:Viscosities.gif">Synapticrelay/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>We wanted to figure out the mechanism behind this surprising result. So we identified what molecules were involved in this process, discovering a cascade of events that allow high viscosity environments to enhance cell motility. </p>
<p>We found that high viscosity fluids first promote the growth of protein filaments called actin, which open channels in the cell’s membrane and increase water intake. The cell expands from the water, activating another channel that takes in calcium ions. These calcium ions activate another type of protein filament called myosin that induces the cell to move. This cascade of events induces cells to change their structure and generate more force to overcome the resistance imposed by high viscosity fluid, meaning the cells aren’t inert at all.</p>
<p>We also discovered that cells retained “memory” after exposure to a high viscosity medium. This meant that if we put cells in a high viscosity medium for several days and then returned them to a normal viscosity medium, they would still move at a faster speed. How cells retain this memory is still an open question.</p>
<p>We then wondered whether our findings on viscous memory would remain true in animals, not just in Petri dishes. So we exposed human breast cancer cells to a high viscosity medium for six days, then placed them in a normal viscosity medium. We then injected the cells into chicken embryos and mice.</p>
<p>Our results were consistent: Cells pre-exposed to a high viscosity medium had an increased ability to leak into surrounding tissues and metastasize compared to cells that were not pre-exposed. This result demonstrates that the viscosity of the fluids in a cell’s surrounding environment is a mechanobiological cue that promotes cancer cells to metastasize.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/OcigJn8UJNQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Understanding how cells move could help elucidate how tumors metastasize.</span></figcaption>
</figure>
<h2>Implications for cancer treatment</h2>
<p>Cancer patients usually don’t die from the original source of the tumor, but from its <a href="https://doi.org/10.1002%2Fcam4.2474">spread to other parts of the body</a>.</p>
<p>When cancer cells travel through the body, they move into spaces that will have varying fluid viscosity. Understanding how fluid viscosity affects the movement of tumor cells could help researchers figure out ways to better treat and detect cancer before it metastasizes. </p>
<p>The next step is to build imaging and analysis techniques to precisely examine how cells from various types of lab animals respond to changes in fluid viscosity. Identifying the molecules that regulate how cells respond to changes in viscosity could help researchers identify potential drug targets to reduce the spread of cancer.</p><img src="https://counter.theconversation.com/content/195792/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Yizeng Li receives funding from National Science Foundation.</span></em></p>Counterintuitively, cells move faster in thicker fluids. New research on breast cancer cells explains why, and reveals the role that fluid viscosity plays in metastasis.Yizeng Li, Assistant Professor of Biomedical Engineering, Binghamton University, State University of New YorkLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1891042022-09-20T12:35:58Z2022-09-20T12:35:58ZDeepfake audio has a tell – researchers use fluid dynamics to spot artificial imposter voices<figure><img src="https://images.theconversation.com/files/484966/original/file-20220915-33289-bmfzf6.jpg?ixlib=rb-1.1.0&rect=818%2C7%2C4057%2C3165&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">With deepfake audio, that familiar voice on the other end of the line might not even be human let alone the person you think it is.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/male-hacker-wearing-mask-while-using-laptop-at-royalty-free-image/1181587363">Knk Phl Prasan Kha Phibuly/EyeEm via Getty Images</a></span></figcaption></figure><p>Imagine the following scenario. A phone rings. An office worker answers it and hears his boss, in a panic, tell him that she forgot to transfer money to the new contractor before she left for the day and needs him to do it. She gives him the wire transfer information, and with the money transferred, the crisis has been averted. </p>
<p>The worker sits back in his chair, takes a deep breath, and watches as his boss walks in the door. The voice on the other end of the call was not his boss. In fact, it wasn’t even a human. The voice he heard was that of an audio deepfake, a machine-generated audio sample designed to sound exactly like his boss.</p>
<p>Attacks like this using recorded audio <a href="https://www.protocol.com/enterprise/deepfake-voice-cyberattack-ai-audio">have already occurred</a>, and conversational audio deepfakes might not be far off.</p>
<p>Deepfakes, both audio and video, have been possible only with the development of sophisticated machine learning technologies in recent years. Deepfakes have brought with them a new level of <a href="https://www.washingtonpost.com/technology/2022/08/30/deep-fake-video-on-agt/">uncertainty around digital media</a>. To detect deepfakes, many researchers have turned to analyzing visual artifacts – minute glitches and inconsistencies – found in <a href="https://theconversation.com/examining-a-videos-changes-over-time-helps-flag-deepfakes-120263">video deepfakes</a>. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/oxXpB9pSETo?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">This is not Morgan Freeman, but if you weren’t told that, how would you know?</span></figcaption>
</figure>
<p>Audio deepfakes potentially pose an even greater threat, because people often communicate verbally without video – for example, via phone calls, radio and voice recordings. These voice-only communications greatly expand the possibilities for attackers to use deepfakes. </p>
<p>To detect audio deepfakes, <a href="https://scholar.google.com/citations?hl=en&user=txlJCGYAAAAJ&view_op=list_works&sortby=pubdate">we and our research colleagues</a> at the University of Florida have developed a technique that <a href="https://www.usenix.org/conference/usenixsecurity22/presentation/blue">measures the acoustic and fluid dynamic differences</a> between voice samples created organically by human speakers and those generated synthetically by computers. </p>
<h2>Organic vs. synthetic voices</h2>
<p>Humans vocalize by forcing air over the various structures of the vocal tract, including vocal folds, tongue and lips. By rearranging these structures, you alter the acoustical properties of your vocal tract, allowing you to create over 200 distinct sounds, or phonemes. However, human anatomy fundamentally limits the acoustic behavior of these different phonemes, resulting in a relatively small range of correct sounds for each.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/SVKR3ESdAk8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">How your vocal organs work.</span></figcaption>
</figure>
<p>In contrast, audio deepfakes are created by first allowing a computer to listen to audio recordings of a targeted victim speaker. Depending on the exact techniques used, the computer <a href="https://dl.acm.org/doi/10.5555/3327345.3327360">might need to listen to as little as 10 to 20 seconds of audio</a>. This audio is used to extract key information about the unique aspects of the victim’s voice. </p>
<p>The attacker selects a phrase for the deepfake to speak and then, using a modified text-to-speech algorithm, generates an audio sample that sounds like the victim saying the selected phrase. This process of creating a single deepfaked audio sample can be accomplished in a matter of seconds, potentially allowing attackers enough flexibility to use the deepfake voice in a conversation.</p>
<h2>Detecting audio deepfakes</h2>
<p>The first step in differentiating speech produced by humans from speech generated by deepfakes is understanding how to acoustically model the vocal tract. Luckily scientists have techniques to estimate what someone – or some being such as a <a href="https://carnegiemnh.org/what-did-dinosaurs-sound-like-paleoacoustics/">dinosaur</a> – would sound like based on anatomical measurements of its vocal tract. </p>
<p>We did the reverse. By inverting many of these same techniques, we were able to extract an approximation of a speaker’s vocal tract during a segment of speech. This allowed us to effectively peer into the anatomy of the speaker who created the audio sample.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/482539/original/file-20220902-22-7jgydo.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="line drawing diagram showing two focal tracts, one wider and more variable than the other" src="https://images.theconversation.com/files/482539/original/file-20220902-22-7jgydo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/482539/original/file-20220902-22-7jgydo.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=342&fit=crop&dpr=1 600w, https://images.theconversation.com/files/482539/original/file-20220902-22-7jgydo.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=342&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/482539/original/file-20220902-22-7jgydo.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=342&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/482539/original/file-20220902-22-7jgydo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=430&fit=crop&dpr=1 754w, https://images.theconversation.com/files/482539/original/file-20220902-22-7jgydo.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=430&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/482539/original/file-20220902-22-7jgydo.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=430&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Deepfaked audio often results in vocal tract reconstructions that resemble drinking straws rather than biological vocal tracts.</span>
<span class="attribution"><a class="source" href="https://www.usenix.org/conference/usenixsecurity22/presentation/blue">Logan Blue et al.</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>From here, we hypothesized that deepfake audio samples would fail to be constrained by the same anatomical limitations humans have. In other words, the analysis of deepfaked audio samples simulated vocal tract shapes that do not exist in people. </p>
<p>Our testing results not only confirmed our hypothesis but revealed something interesting. When extracting vocal tract estimations from deepfake audio, we found that the estimations were often comically incorrect. For instance, it was common for deepfake audio to result in vocal tracts with the same relative diameter and consistency as a drinking straw, in contrast to human vocal tracts, which are much wider and more variable in shape.</p>
<p>This realization demonstrates that deepfake audio, even when convincing to human listeners, is far from indistinguishable from human-generated speech. By estimating the anatomy responsible for creating the observed speech, it’s possible to identify the whether the audio was generated by a person or a computer.</p>
<h2>Why this matters</h2>
<p>Today’s world is defined by the digital exchange of media and information. Everything from news to entertainment to conversations with loved ones typically happens via digital exchanges. Even in their infancy, deepfake video and audio undermine the confidence people have in these exchanges, effectively limiting their usefulness.</p>
<p>If the digital world is to remain a critical resource for information in people’s lives, effective and secure techniques for determining the source of an audio sample are crucial.</p><img src="https://counter.theconversation.com/content/189104/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Logan Blue receives funding from the Office of Naval Research for this work.. </span></em></p><p class="fine-print"><em><span>Patrick Traynor receives funding from the Office of Naval Research for this work.</span></em></p>AI-generated voice-alikes can be indistinguishable from the real person’s speech to the human ear. A computer model that gives voice to the dinosaurs turns out to be a good way to tell the difference.Logan Blue, PhD candidate in Computer & Information Science & Engineering, University of FloridaPatrick Traynor, Professor of Computer and Information Science and Engineering, University of FloridaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1637212021-07-22T20:05:15Z2021-07-22T20:05:15ZThe science of underwater swimming: how staying submerged gives Olympians the winning edge<figure><img src="https://images.theconversation.com/files/412390/original/file-20210721-13-1gmnwji.jpg?ixlib=rb-1.1.0&rect=35%2C0%2C2595%2C1329&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>To win swimming gold in Tokyo, swimmers not only have to generate incredible power with their arms and legs to propel themselves through the water; they also have to overcome the relentless pull of the water’s drag while doing so.</p>
<p>Without being able to don special low-drag suits or use technologies to help them fly over the water, how can swimmers make the effect of the water’s drag as small as possible?</p>
<p>The best athletes in this year’s Olympics will do it by swimming under, rather than on top of, the water – at least as far as the rules allow.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/fast-suits-and-olympic-swimming-a-tale-of-reduced-drag-and-broken-records-7960">'Fast suits' and Olympic swimming: a tale of reduced drag and broken records</a>
</strong>
</em>
</p>
<hr>
<h2>Waves are a drag</h2>
<p>Water is much denser than air, so you might assume swimmers would benefit from using a technique that allows them to sit high in the water, with as much of their body out of the water as possible.</p>
<p>But there are two problems with this strategy. </p>
<p>First, it costs energy to produce the forces needed to lift the body, which would be better spent propelling the swimmer forwards towards the finishing wall.</p>
<p>Second, when we travel on the water’s surface we waste energy making waves. During fast swimming, such as in the sprint freestyle events or during starts and turns (where speeds exceed 2 metres per second, or about 7 kilometres per hour), wave generation slows the swimmer down more than any other factor. Reducing wave formation is therefore vital to swimming success.</p>
<p>Waves are produced as the pressure exerted by the swimmer on the water forces the water upwards and out of their path. Other pressure changes around the swimmer’s body also cause waves to form behind them, and sometimes to the side.</p>
<p>The energy required to generate waves comes from the swimmer themselves, so a lot of the power generated by the swimmer’s muscles is used in wave generation rather than moving the swimmer forwards. </p>
<p>But waves aren’t formed when we (or fish, dolphins or whales) swim under the water, because waves only form when an object (like us) moves at the boundary between two fluids of different densities, such as water and air during swimming. And this fact hints at an intriguing solution to the drag issue.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/tokyo-olympics-what-are-the-limits-of-human-performance-podcast-164882">Tokyo Olympics: what are the limits of human performance? Podcast</a>
</strong>
</em>
</p>
<hr>
<h2>A change in thinking</h2>
<p>Swimmers had noticed the benefits of staying underwater from at least the 1950s. </p>
<p>The breaststroke event was the cause of major controversy in the 1956 Melbourne Olympic Games as swimmers experimented with staying underwater for much of their races. The winner of the men’s 200-metre event, Masaru Furukawa of Japan, swam underwater for most of the first three laps of the four-lap race. This practice was swiftly outlawed after the games; swimmers were forced to surface before they could start to swim.</p>
<p>But the practice of swimming underwater in freestyle (front crawl), butterfly and backstroke events only took off after swimmers mastered the “underwater undulatory technique”, better known as the dolphin kick. </p>
<p>Here, the swimmer propels themselves underwater by undulating the lower body in a wave-like manner while maintaining a rigid and streamlined upper body position with arms stretched overhead.</p>
<p>The amplitude of the lower body undulation increases from the hips to the feet so the “wave” produced by the body is much greater down towards the feet, creating a whip-like effect. This pushes water rapidly backwards, propelling the swimmer forwards according to Newton’s law of action and reaction.</p>
<p>Using this technique, swimmers in the backstroke events gained a significant advantage from the 1980s onwards, and from the 1990s it was also common in freestyle and butterfly events.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Vox9KOxC1ZA?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Underwater swimming speeds can far exceed normal swimming speeds.</span></figcaption>
</figure>
<p>The technique was so effective that swimming’s governing body, FINA, limited its use to only the 15-metre segment after starts and turns. Swimmers are now disqualified if they swim too far underwater.</p>
<p>Yet the benefits of improving the underwater undulation technique are so great that swimmers still spend hours each week in training improving this part of the race. </p>
<h2>Keys to underwater swimming success</h2>
<p>Although an ongoing research effort aims to find the optimum technique for different swimmers, a few practices seem to be commonly associated with underwater success.</p>
<p>First, swimmers who stay underwater for the full 15 metres will have <a href="https://ojs.ub.uni-konstanz.de/cpa/article/view/3870">faster starts</a>, turns and overall race times. This <a href="https://pubmed.ncbi.nlm.nih.gov/26930126/">effect</a> is particularly strong in backstroke events, and when swimmers make the most of the final turn in a race (when swimmers usually surface quicker because they are growing tired).</p>
<p>Second, staying deeper underwater is important. Wave drag is slightly reduced by swimming just below the surface, but swimming 40–60 centimetres underwater can <a href="https://swimmingcoach.org/pdf/pub/jsr1998.pdf#page=19">reduce drag by 10–20%</a>. And there are further benefits when swimming <a href="https://pubmed.ncbi.nlm.nih.gov/25555171/">a metre</a> or more under the water, especially when start and turn push-off speeds <a href="https://pubmed.ncbi.nlm.nih.gov/16439236/">are fast</a> (as in most shorter races). </p>
<p>Third, the best swimmers will likely display a <a href="https://memberdesq.sportstg.com/assets/console/customitem/attachments/Underwater%20Undulatory%20Swimming%20Applications%20-%20R%20Arellano.pdf">faster kick frequency</a>, although each kick is no bigger than those of slower swimmers. In particular, a <a href="https://pubmed.ncbi.nlm.nih.gov/26367778/">fast extension of the knee</a> in the downbeat of the kick that occurs at the end of the wave-like motion may separate the faster from slower underwater swimmers.</p>
<p>And finally, although it will be hard to spot in the underwater camera shots at the Olympics, the feet of the faster underwater swimmers may <a href="https://pubmed.ncbi.nlm.nih.gov/31216935/">rotate inwards</a> during the downbeat of the kick, rather than staying rigidly in line with the leg. This rotation allows the top surfaces of the feet to orient horizontally to the swimming direction, just like the flute (tail) of a dolphin or whale lies horizontal to their swim direction, producing more propulsion at the feet.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/vY6GxQqAkuQ?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Foot rotation during underwater swimming may be key to power production (clear example at 5:00 minutes).</span></figcaption>
</figure>
<h2>Submarining towards gold</h2>
<p>So, at the Tokyo Olympic Games, look for the swimmers who stay underwater as long as allowed in starts and turns, and check the techniques they use when the director cuts to the underwater shots. </p>
<p>The swimmers who make the most of these parts of the race might just propel themselves to Olympic gold.</p><img src="https://counter.theconversation.com/content/163721/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Anthony Blazevich does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Swimming underwater reduces drag so much there are rules against doing it for too long. But the best swimmers make the most of what’s allowed.Anthony Blazevich, Professor of Biomechanics, Edith Cowan UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1539722021-01-27T13:26:04Z2021-01-27T13:26:04ZExpert in fluid dynamics explains how to reduce the risk of COVID-19 airborne transmission inside a car<figure><img src="https://images.theconversation.com/files/380587/original/file-20210125-15-18tvsyd.jpg?ixlib=rb-1.1.0&rect=659%2C366%2C7491%2C5377&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Opening all windows, or one front and one rear window, increases the amount of airflow in the car, reducing the risk of airborne transmission.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/young-woman-wearing-disposable-face-mask-while-royalty-free-image/1214326607?adppopup=true">Sisoje/E+ via Getty Images</a></span></figcaption></figure><p><em>Editor’s note: Varghese Mathai is a physicist at the University of Massachusetts Amherst who studies the flow of fluids and gases. He conducted a <a href="http://dx.doi.org/10.1126/sciadv.abe0166">study</a> using computational fluid dynamics simulations to understand how air flows inside a car and its implications for COVID-19 airborne transmission. In <a href="https://youtu.be/J7EAVIHpFhE">this interview</a>, he explains the optimal ways to ensure maximum airflow inside a car.</em></p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/J7EAVIHpFhE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Varghese Mathai of the University of Massachusetts Amherst explains and shows how air flows inside cars and how to lower the risk of COVID-19 airborne transmission.</span></figcaption>
</figure>
<h2>What can be done to reduce the risk of airborne transmission inside a car?</h2>
<p>It’s important to have good ventilation. This means you get as much outside air as possible to mix with the air inside the cabin and then flush it out. </p>
<p>You can do this in a couple of ways. One is by turning on the heating system, which takes in fresh air from outside, and opening windows through which it can be flushed out. Another way is to just have the windows open. The benefit of having windows open is that if you are riding at 20 miles an hour or faster, a lot of air is flushed out just by the speed of the car. </p>
<p>Having windows open allows more air to be flushed out than by just having the heating or air conditioning turned on.</p>
<h2>Which windows should be kept open and closed to ensure optimal airflow?</h2>
<figure>
<img src="https://cdn.theconversation.com/static_files/files/1452/Car_flow_gif.gif?1611625501">
<figcaption><span class="caption">Computational fluid dynamics simulation of airflow in cars with two windows, one rear, one front, open. Source: Varghese Mathai CC-BY-ND</span></figcaption>
</figure>
<p>We think the best configuration is to have all windows open, and if possible fully open. If this is not practical, then it would be good to have two windows open. Preferably one in the rear and one in in the front. </p>
<p>What we found out from computer simulations is that the air enters through the rear window, turns around behind the rear passenger and exits through the front window. This way, many of these aerosol particles inside the cabin can be flushed out. </p>
<h2>What about barriers and screens between the passenger and the driver?</h2>
<p>Many taxis and ride-share services like Uber and Lyft have been using a barrier or a screen between the front and rear areas of the cabin. These help in reducing transmission through larger droplets. These are the kinds of <a href="https://www.cdc.gov/coronavirus/2019-ncov/more/scientific-brief-sars-cov-2.html">droplets</a> that are released through coughing, sneezing or talking loudly. Decontaminating surfaces helps against the <a href="https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions">fomite transmission</a>. But <a href="https://www.cdc.gov/coronavirus/2019-ncov/more/scientific-brief-sars-cov-2.html">airborne</a> transmission would not be reduced much by these barriers because there are always gaps and holes in barriers through which air can pass through.</p>
<h2>How did you conduct this study?</h2>
<p>For this study we used computer simulations, specifically computational fluid dynamics simulations, which are widely used for studying flows around cars and airplanes. We used it because of its quick turnaround time, so that we could compare different open and closed window configurations and qualitatively predict which might be better in terms of removing these airborne particles. </p>
<figure>
<img src="https://cdn.theconversation.com/static_files/files/1453/car_wand_gif.gif?1611625878">
<figcaption><span class="caption">Footage from one of the field tests conducted to understand airflow in a car. Source: Varghese Mathai CC-BY-ND</span></figcaption>
</figure>
<p>After this publication was out we did go in and do a number of field tests to get some kind of validation of the airflows that were simulated. We released smoke at different locations inside the car and looked at the pathways of the smoke as it was released inside the car. It was more or less the same as what we found from the computer simulations.</p><img src="https://counter.theconversation.com/content/153972/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Varghese Mathai does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Keeping windows open while driving at a moderate speed can increase airflow inside the cabin of the car, but which ones should you keep open?Varghese Mathai, Assistant Professor of Physics, UMass AmherstLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1422332020-07-09T12:14:17Z2020-07-09T12:14:17ZAerosols are a bigger coronavirus threat than WHO guidelines suggest – here’s what you need to know<figure><img src="https://images.theconversation.com/files/346474/original/file-20200708-3974-kkocl9.jpg?ixlib=rb-1.1.0&rect=7%2C247%2C4823%2C3375&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Aerosols are made up of tiny respiratory droplets suspended in the air.</span> <span class="attribution"><a class="source" href="http://gettyimages.com/detail/photo/man-sneezing-royalty-free-image/94989536">Jeffrey Coolidge via Getty Images</a></span></figcaption></figure><p><em>Leer en <a href="https://theconversation.com/que-son-los-aerosoles-y-por-que-son-tan-peligrosos-ante-la-pandemia-de-covid-19-143515">español</a></em></p>
<p>When someone coughs, talks or even breathes, they send tiny respiratory droplets into the surrounding air. The smallest of these droplets can float for hours, and there is strong <a href="https://doi.org/10.1038/s41586-020-2271-3">evidence</a> that they can <a href="https://dx.doi.org/10.3201/eid2606.200412">carry live coronavirus</a> if the person is infected.</p>
<p>Until mid-July, however, the risk from these aerosols wasn’t incorporated into the World Health Organization’s <a href="https://www.who.int/publications/i/item/advice-on-the-use-of-masks-in-the-community-during-home-care-and-in-healthcare-settings-in-the-context-of-the-novel-coronavirus-(2019-ncov)-outbreak">formal guidance</a> for nations. The WHO instead suggested that the coronavirus was primarily transmitted by coughing or sneezing large droplets into someone’s face, rather than being a longer-term threat that can float in the air.</p>
<p>It took pressure from scientists to start to change that.</p>
<p>More than 200 scientists published an open letter to the WHO on July 6 warning about airborne transmission of COVID-19 via aerosols and urging the organization to recognize the risks. The <a href="https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions">WHO responded</a> Thursday afternoon with an update in which it acknowledged the growing evidence of airborne spread of the disease, but it did so with hesitation.</p>
<p>As <a href="https://www.clarkson.edu/people/andrea-ferro">professors</a> <a href="https://www.clarkson.edu/people/byron-erath">who study</a> <a href="https://www.clarkson.edu/people/goodarz-ahmadi">fluid dynamics</a> and aerosols, we believe it is important for people to understand the risks and what they can do to protect themselves.</p>
<h2>What is an aerosol and how does it spread?</h2>
<p>Aerosols are particles that are suspended in the air. When humans breathe, talk, sing, cough or sneeze, the emitted respiratory droplets mix in the surrounding air and form an aerosol. Because larger droplets quickly fall to the ground, respiratory aerosols are often described as being made up of smaller droplets that are less than 5 microns, or about one tenth the width of a human hair. </p>
<p>In general, droplets form as a sheet of liquid breaks apart. You’ve probably experienced this phenomenon by blowing soap bubbles. Sometimes the bubble doesn’t fully form, but instead breaks apart into many droplets. </p>
<p>Similarly, in humans, small sheets and strands of liquid – mucus – often stretch across portions of the airway. This most often occurs in locations where the airway opens and closes again and again. That happens deep within the lungs as the bronchioles and alveolar sacs expand and contract during breathing, within the larynx as the vocal folds vibrate during speech, or at the mouth, as the tongue and lips move while talking. The airflow produced by breathing, speaking and coughing breaks apart these sheets of mucus, just like blowing the soap bubble. </p>
<p>[<em>Deep knowledge, daily.</em> <a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=deepknowledge">Sign up for The Conversation’s newsletter</a>.]</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/piCWFgwysu0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Slow-motion video of a sneeze shows suspended droplets. Credit: Prof. L Bourouiba/JAMA Network https://bit.ly/3exRYc3.</span></figcaption>
</figure>
<p>The size of the droplets varies based on how and where they are produced within the airway. While coughing generates the largest quantity of droplets, <a href="https://doi.org/10.1038/s41598-019-38808-z">research has shown</a> that just two to three minutes of talking can produce as many droplets as one cough.</p>
<p>Droplets that are smaller than 5 microns <a href="https://doi.org/10.1371/journal.pone.0021481">can remain suspended in the air for many minutes to hours</a> because the effect of air drag relative to gravity is large. In addition, the water content of virus-carrying droplets evaporates while they are airborne, decreasing their size. Even if most of the fluid evaporates from a virus-laden droplet, the droplet does not disappear; it just becomes smaller, and the smaller the droplet, the longer it will stay suspended in the air. Because smaller diameter droplets are more <a href="http://journals.sagepub.com/doi/abs/10.1260/1757-482X.4.2.159">efficient at penetrating deep into the pulmonary system</a>, they also pose a much greater infection risk. </p>
<p>The <a href="https://reliefweb.int/report/world/advice-use-masks-context-covid-19-interim-guidance-5-june-2020">WHO guidelines</a> suggested that the virus RNA found in small droplets wasn’t viable in most circumstances. However, early research on the SARS-CoV-2 virus has shown that it is <a href="https://www.nejm.org/doi/full/10.1056/nejmc2004973">viable as an aerosol for up to 3 hours</a>.</p>
<h2>Do masks protect from aerosol transmission?</h2>
<p>Face coverings and masks are absolutely necessary for protection against aerosol transmission. They serve a twofold purpose. </p>
<p>First, they filter the air expelled by an individual, capturing respiratory droplets and thereby reducing the exposure risk for others. This is particularly important as they are most effective at capturing larger droplets that are more likely to have larger quantities of viruses encapsulated within them. This prevents the larger droplets from directly affecting someone, or evaporating down to a smaller size and circulating in the air. </p>
<p>They also <a href="https://www.youtube.com/watch?v=9Mkb4TMT_Cc&feature=youtu.be">reduce the speed</a> of the puff of air that is produced when sneezing, coughing or talking. Decreasing the velocity of the expelled air reduces the distance that droplets are initially transported into the person’s surroundings. </p>
<p>It is important to realize, however, that the protection provided by masks and face coverings varies depending on the material they are constructed from and how well they fit. Nevertheless, <a href="https://theconversation.com/why-wear-face-masks-in-public-heres-what-the-research-shows-135623">wearing face coverings</a> to decrease airborne exposure risk is critical.</p>
<h2>Is staying 6 feet away enough to stay safe?</h2>
<p>The recommendation to maintain a 6-foot separation is based on a <a href="https://doi.org/10.1093/oxfordjournals.aje.a118097">study by W. F. Wells in 1934</a> that showed an expelled water droplet either falls to the ground, or evaporates, within a distance of roughly 2 meters, or 6 feet. The study did not, however, account for the fact that following evaporation of the water in a virus-laden droplet, <a href="https://www.ncbi.nlm.nih.gov/books/NBK143281/">the droplet nuclei remains</a>, thereby still posing a risk of airborne infection.</p>
<p>Consequently, while staying 6 feet from other people reduces exposure, it might not be sufficient in all situations, <a href="https://doi.org/10.1101/2020.04.16.20067728">such as in enclosed, poorly ventilated rooms</a>.</p>
<h2>How can I protect myself from aerosols indoors?</h2>
<p>Strategies to mitigate airborne exposure are similar to strategies for staying dry when it’s raining. The longer you stay in the rain, and the harder it’s raining, the wetter you will get. Similarly, the more droplets you are exposed to, and the longer you stay in that environment, the higher the exposure risk. Mitigating risk is therefore based on decreasing both aerosol concentration levels and exposure time. </p>
<p>Aerosol concentrations can be reduced with increased ventilation, although recirculating the same air should be avoided unless the air can be effectively filtered prior to reuse. When possible, open doors and windows to increase fresh air flow. </p>
<p>Decreasing the number of emission sources – people – within a space, and ensuring that face coverings are worn at all times can further decrease concentration levels. </p>
<p>Methods of deactivating the virus, such as <a href="https://doi.org/10.1213/ANE.0000000000004829">germicidal ultraviolet light</a>, can also be used.</p>
<p>Finally, reducing the amount of time you spend in poorly ventilated, crowded areas is a good way to reduce airborne exposure risk. </p>
<p><em>This article has been updated with the WHO response.</em></p>
<p><em><a href="https://scholar.google.com/citations?user=HWAlZA8AAAAJ&hl=en">Amir Mofakham</a>, a research associate in mechanical engineering at Clarkson University, contributed to this article.</em></p><img src="https://counter.theconversation.com/content/142233/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Byron Erath receives funding from the National Science Foundation and the National Institutes of Health. He is affiliated with the American Society of Mechanical Engineers, and the American Physical Society, Division of Fluid Dynamics.</span></em></p><p class="fine-print"><em><span>Andrea Ferro receives funding from the National Science Foundation. She is affiliated with the American Association for Aerosol Research, the International Society of Indoor Air Quality and Climate, and the International Society for Exposure Analysis. </span></em></p><p class="fine-print"><em><span>Goodarz Ahmadi receives funding from the National Science Foundation. He is a Fellow of American Society of Mechanical Engineers (ASME), and also a Fellow of American Society of Thermal and Fluid Engineers (ASTFE). He is life member of International Society for Porous Media (InterPore).</span></em></p>More than 200 scientists wrote to the World Health Organization, warning about aerosol transmission of the coronavirus.Byron Erath, Associate Professor of Mechanical Engineering, Clarkson UniversityAndrea Ferro, Professor of Civil & Environmental Engineering, Clarkson UniversityGoodarz Ahmadi, Professor of Mechanical Engineering, Clarkson UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1421232020-07-09T07:17:24Z2020-07-09T07:17:24ZWhy it’s important to know how fluids move<figure><img src="https://images.theconversation.com/files/346341/original/file-20200708-3991-1rr7ktw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Knowing how fluids move can help us understand virus transmission better.</span> <span class="attribution"><span class="source">Stock image/Getty Images</span></span></figcaption></figure><p>COVID-19 has sharply brought into focus the impact of respiratory infectious diseases on humanity. It has also shown that the greatest defence against health crises is government policy that is strongly rooted in science.</p>
<p>In an attempt to contain the spread of the disease many governments have implemented policies that prevent physical interaction. This has a direct impact on our daily lives: the way people move, where they work and how they interact with each other. </p>
<p>These policies are a good first step, but they are limited because they don’t fully account for how the virus physically spreads from person to person – that is, the physical routes of transmission. Some of the answers may lie in a field of study called fluid mechanics – understanding how fluids move.</p>
<p>Understanding how fluids move can help us get to grips with how a virus like the one causing COVID-19 travels from an infected person to others. This is because when we cough or sneeze we expel micro-droplets whose motion is governed by the principles of fluid mechanics. Understanding the way the virus is transmitted can inform public health interventions to minimise the risk.</p>
<p>Recent insights at the interface between fluid mechanics and epidemiology are already beginning to unlock at least some understanding of COVID-19’s physical routes of transmission. <a href="https://jamanetwork.com/journals/jama/fullarticle/2763852">For example</a>, recent work has shown that a cough or sneeze consists of a multiphase chaotic gas cloud. This gas cloud transports viral pathogens much further than predicted. </p>
<p>The mounting evidence has led the World Health Organisation (WHO) to <a href="https://www.bbc.com/news/world-53329946">acknowledge</a> that the coronavirus can be spread by tiny particles suspended in the air.</p>
<h2>The study of fluid mechanics</h2>
<p><a href="https://www.britannica.com/science/fluid-mechanics">Fluid mechanics</a> is the study of how fluids move. That may sound simple, but it’s actually very complex.</p>
<p>First, it’s important to understand what it means to move. The physicist <a href="https://www.britannica.com/biography/Isaac-Newton">Sir Isaac Newton</a> showed that something called a force is required to change how an object moves. This force must be applied to the object and the magnitude and direction of this force is the product of the object’s mass and acceleration. </p>
<p>Acceleration refers to how something’s velocity changes with time (rate of change of velocity). Additionally, an object’s velocity refers to the distance it travels in a certain amount of time. Therefore, Newton’s laws of motion can help us predict how an object moves through space and time. This helps us calculate the object’s position at any given time.</p>
<p>We can apply Newton’s <a href="https://www.sciencedirect.com/topics/engineering/newtons-second-law">laws of motion to fluids</a> in an attempt to explain how fluids move. A fluid is a substance whose particles move a lot relative to each other when a force is applied to it.</p>
<p>In other words the defining property of a fluid is the ease with which the fluid can be deformed. Fluids have no defined shape. Any liquid such as water is a good example, but the air around us can also be treated as a fluid. The line between fluids and solids is not distinct and in some instances solid objects such as jelly, dry paint and asphalt can behave like fluids and <a href="https://fyfluiddynamics.com/2012/09/many-common-fluidslike-air-and-waterare/">vice versa</a>. </p>
<p>An important characteristic of fluids is that they transport “things”; these “things” could be heat, pollutants, pathogens or other fluids. Therefore the study of fluid mechanics is fundamental to understanding the world we live in. For example fluid mechanics can help us model and predict how heat from the sun gets transported around the world (think climate change). Another example is the applications of fluid mechanics to the transport of respiratory diseases such as COVID-19.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/is-the-airborne-route-a-major-source-of-coronavirus-transmission-141198">Is the airborne route a major source of coronavirus transmission?</a>
</strong>
</em>
</p>
<hr>
<h2>Applying fluid mechanics to COVID-19</h2>
<p><a href="https://doi.org/10.1017/jfm.2020.330">Recent studies</a> have shown that our current understanding of the routes of transmission of respiratory diseases is limited and based on an oversimplified model. <a href="https://doi.org/10.1017/jfm.2014.88">Recent developments</a> in fluid mechanics and epidemiology have shown that turbulent puffs, emitted by sneezing or coughing, transport pathogens much further than expected. </p>
<p>The characteristics of the “puff” or “plume” that is emitted when we breathe, cough or sneeze are important for understanding how the fluid droplets are transported. </p>
<p>Fluid droplets inside the “puff” get distorted by the complex air flow patterns associated with the “puff” or “plume” and its interaction with the ambient air. This process can break up a fluid droplet into several pieces that fall out of suspension, contaminating many surfaces. Of course the break-up process can also result in smaller particles that can travel further than the larger ones. </p>
<p>It’s also been shown that the flow field, temperature and humidity affect how far these droplets travel. This has implications for the WHO’s 1-2m (3-6 ft) physical distancing <a href="https://www.who.int/emergencies/diseases/novel-coronavirus-2019/advice-for-public">guidelines</a>. Research has <a href="https://doi.org/10.1017/jfm.2014.88">shown</a> that these droplets can travel as far as 7m (20ft). This doesn’t take into account the micro-droplets that can be transported even further because of building ventilation systems. </p>
<p>In an open letter to the WHO more than 200 scientists <a href="https://www.bbc.com/news/world-53329946">recently accused</a> the organisation of underestimating the possibility of airborne transmission of COVID-19. Given the WHO’s <a href="https://www.bbc.com/news/world-53329946">recent statement</a>, these guidelines are likely to be revised.</p>
<p>The principles of fluid mechanics are <a href="https://books.google.co.za/books?id=Z94ilYqgmy4C&pg=PR16&lpg=PR16&dq=fluid+mechanics+design+occupational+health&source=bl&ots=WG0UVrM_rw&sig=ACfU3U0MS2aYVtvclGencf3ww80OWpa6Yw&hl=en&sa=X&ved=2ahUKEwjsn7T1_rzqAhWsTBUIHZa9CDUQ6AEwA3oECAYQAQ#v=onepage&q=fluid%20mechanics%20design%20occupational%20health&f=false">already used</a> in ventilation design and occupational health; it’s possible that our new understandings about fluid mechanics and epidemiology could be used to help improve building ventilation systems. What we’re learning may also inform government policy to reduce the spread of future pandemics like COVID-19.</p><img src="https://counter.theconversation.com/content/142123/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Justin Pringle receives funding from the National Research Foundation of South Africa. </span></em></p>Fluid mechanics can be applied to the transport of respiratory diseases such as COVID-19.Justin Pringle, Lecturer in Environmental Fluid Mechanics, University of KwaZulu-NatalLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1276732019-11-25T21:49:08Z2019-11-25T21:49:08ZContrary to recent reports, Jupiter’s Great Red Spot is not in danger of disappearing<figure><img src="https://images.theconversation.com/files/303380/original/file-20191125-74576-1fzpz6q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Measuring in at 10,159 miles (16,350 kilometers) in width (as of April 3, 2017) Jupiter’s Great Red Spot is 1.3 times as wide as Earth. </span> <span class="attribution"><a class="source" href="https://www.nasa.gov/image-feature/jpl/pia21774/jupiter-s-great-red-spot-swallows-earth">NASA/JPL-Caltech/SwRI/MSSS/Christopher Go</a></span></figcaption></figure><p>In the last 10 years, but in the last five months in particular, <a href="https://www.sciencetimes.com/articles/22314/20190603/amateur-astronomer-discovers-jupiters-great-red-spot-dying.htm">the press has reported dire warnings</a> that the <a href="https://www.cbc.ca/news/technology/jupiter-great-red-spot-1.5154387">Great Red Spot of Jupiter is dying</a>. However, some astronomers believe, to paraphrase Mark Twain, that the reports of its death are greatly exaggerated, or at least premature.</p>
<p>Robert Hooke, an early British physicist who discovered cells, <a href="https://www.jstor.org/stable/101402">first described the Great Red Spot in 1665</a>. In 1979, when two <a href="https://voyager.jpl.nasa.gov/">Voyager</a> spacecraft flew close by Jupiter, images showed that the spot was a red cloud that rotated as part of a huge vortex several times larger than the Earth.</p>
<p>Concerns for the Great Red Spot’s “health” arose when astronomers realized that the cloud’s area in 1979 was only half of its size in the 1800s, as determined from old photographic plates. Recent images showed <a href="https://doi.org/10.1006/icar.2002.6867">more cloud shrinkage</a>, leading to headlines that the spot could die within 20 years. In spring 2019, astronomers reported that it was “unraveling,” and shedding large “blades” and “flakes” of red clouds. </p>
<p><a href="https://cfd.me.berkeley.edu/people/philip-marcus/">I</a> have been intrigued by the Great Red Spot since 1979, when I viewed the Voyager images only days after NASA processed them. The beautiful structure of this extraordinary atmospheric intrigued me since my <a href="https://cfd.me.berkeley.edu/list-of-publications/">career was evolving from astrophysics to fluid dynamics</a> – the study of how liquids and gases move. What better way to begin exploring the fundamental physics and math of fluid dynamics than to study the Great Red Spot?</p>
<h2>Jupiter’s clouds and vortices</h2>
<p>I believe that the <a href="http://meetings.aps.org/Meeting/DFD19/Session/L13.1">Great Red Spot is in no danger of disappearing</a>. By analyzing the cloud images with computer models that incorporate the physics of how fluids move, my research group at Berkeley was able to <a href="http://doi.org/doi:%2010.1016/j.icarus.2010.06.026">determine the area</a> of the spot. We discovered that the area of the spot cloud is larger than its underlying vortex, the swirling gas that defines it. The question then becomes: Does a decrease in the area of the cloud mean that the vortex itself is shrinking? </p>
<p>It is difficult to determine the relationship between the cloud’s size and the vortex’s size or even how Jovian clouds form and dissipate. Therefore, to understand the health of the spot, planetary scientists need to study the health of its vortex and not its cloud; the cloud’s shrinkage is not a harbinger of death. Based on the spot’s interactions with other vortices my Berkeley group found there is no evidence that that vortex itself has changed its size or intensity.</p>
<p>Jupiter’s atmosphere contains vortices besides the Great Red Spot, some of which are useful for monitoring its health. Some, like this spot, are anticyclones that rotate in the opposite direction of the planet’s spin; others are cyclones that rotate in the same direction as the planet’s spin. Anticyclones appear as bright clouds and so are easily detectable, but cyclones (except at the poles) often have filamentary clouds or no clouds at all. </p>
<p>How do we know that Jovian cyclones exist when clouds are not visible? For more than a century astronomers documented the motions of cloud-covered anticyclones as they slowly drifted across Jupiter. Changes in their speeds were often abrupt and seemed to occur for no reason. However, by assuming that these observable vortices interact with cloud-free (and unobservable) cyclones, we can explain the abrupt changes.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=377&fit=crop&dpr=1 600w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=377&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=377&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=474&fit=crop&dpr=1 754w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=474&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/303379/original/file-20191125-74572-1vhisqe.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=474&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A (false color) series of images capturing the repeated flaking of red clouds from the GRS in the Spring of 2019. In the earliest image, the flaking is predominant on the east side of the giant red vortex. The flake then breaks off from the GRS, but a new flake starts to detach in the fifth image.</span>
<span class="attribution"><span class="source">Chris Go</span></span>
</figcaption>
</figure>
<h2>Two simultaneous events that led to flaking</h2>
<p>Anticyclones merge with each other. However, anticyclones repel cyclones. In spring 2019, when the “flaking” was observed, the Great Red Spot was also observed to merge with a series of small clouds (likely small anticyclones) on its northwest side. Such mergers are common; Voyager 1 first observed these and they have subsequently been observed every few months. Typically, small anticyclones are not “digested” immediately, but produce lumps on the spot’s boundary that orbit around it, slowly migrating into the center. </p>
<p>I believe that the shedding of clouds from the spot as “flakes” and “blades” observed in 2019 was due to two simultaneous events: undigested lumps of merged anticyclones traveling along the spot’s boundary and a close encounter with one or more “unobservable” cyclones. </p>
<p>When a large anticyclone and smaller cyclone approach each other before repelling, they create a “stagnation” point near the boundary of the anticyclone where the local winds abruptly change direction, going off approximately perpendicular to their original directions. Think of two fire hoses aimed at each other so that their streams of water collide – the streams momentarily halt at the point of impact (the stagnation point) and then scatter outward. Any cloud or undigested lump on the spot that encounters a stagnation point will similarly shatter and flake away in opposite directions.</p>
<p>The numerical calculations of my Berkeley research group show that the recent observations of cloud shedding can be explained by the collision of undigested red clouds at the edge of the Great Red Spot with stagnation points produced during a close encounter with a cyclone. </p>
<p>Pieces of the red cloud scatter outward from the stagnation point, appearing as flakes and blades. Neither the mergers that created the lumps nor the close encounters with cyclones are unusual by themselves, but it is not that common for them to occur at the same time. However, neither event is a sign of ill health for the Great Red Spot. My colleagues and I believe it will survive for many more years.</p>
<p>[ <em><a href="https://theconversation.com/us/newsletters?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=expertise">Expertise in your inbox. Sign up for The Conversation’s newsletter and get a digest of academic takes on today’s news, every day.</a></em> ]</p><img src="https://counter.theconversation.com/content/127673/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Philip Marcus received funding from NASA and NSF. </span></em></p>Little bits of Jupiter’s Great Red Spot seem to be flaking off. Is it a sign of the demise of this enigmatic red cloud, or just a consequence of atmospheric chaos we can’t see from above?Philip Marcus, Professor of Mechanical Engineering, University of California, BerkeleyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1203082019-10-28T14:52:39Z2019-10-28T14:52:39ZCurious Kids: how do ripples form and why do they spread out across the water?<figure><img src="https://images.theconversation.com/files/298940/original/file-20191028-113987-ptaspz.jpg?ixlib=rb-1.1.0&rect=0%2C120%2C3280%2C2106&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The ripple effect.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/detail-water-drop-splash-280061825">Forance/Shutterstock. </a></span></figcaption></figure><p><strong>When I was playing “splash rocks”, I noticed that when I threw the rock into the river it made a circle shape, which got bigger. How does it make the ripple? Why do the circles spread out further and further? Why do they stop? – Rowan, aged six, UK.</strong> </p>
<p>Hi Rowan, these are good questions, and a fun experiment to do. </p>
<p>When you throw a rock into a river, it pushes water out of the way, making a ripple that moves away from where it landed. As the rock falls deeper into the river, the water near the surface rushes back to fill in the space it left behind. </p>
<p>The water usually rushes back too enthusiastically, causing a splash – and the bigger the rock, the bigger the splash. The splash then creates even more ripples that tend to move away from where the rock went into the water.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/RRPP73QM_4k?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>When water is in its calmest, lowest energy state, it has a flat surface. By throwing the rock into the river, you have given the water some energy. That causes the water to move around, trying to spread out the energy so it can go back to having a still, flat surface. </p>
<p>This follows a powerful <a href="https://en.wikipedia.org/wiki/Principle_of_minimum_energy">principle of physics</a>, which is that everything seeks to find a state where its energy is as small as possible.</p>
<hr>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=293&fit=crop&dpr=1 600w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=293&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=293&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=368&fit=crop&dpr=1 754w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=368&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/282267/original/file-20190702-126345-1np1y7m.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=368&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption"></span>
</figcaption>
</figure>
<p><em><a href="https://theconversation.com/au/topics/curious-kids-36782">Curious Kids</a> is a series by <a href="https://theconversation.com/uk">The Conversation</a>, which gives children the chance to have their questions about the world answered by experts. If you have a question you’d like an expert to answer, send it to <a href="mailto:curiouskids@theconversation.com">curiouskids@theconversation.com</a>. We won’t be able to answer every question, but we’ll do our very best.</em></p>
<hr>
<p>One way energy can move around is by forming waves. For example, the waves you see at the beach <a href="https://theconversation.com/curious-kids-why-are-there-waves-112015">are formed</a> by energy from the wind. Light and sound <a href="https://theconversation.com/curious-kids-is-everything-really-made-of-molecules-109145">also move in waves</a>, though we can’t see that directly. And the ripples that you see in the river are small waves carrying away the energy from where you threw the rock.</p>
<h2>Up and down</h2>
<p>You might already know that <a href="https://theconversation.com/curious-kids-is-everything-really-made-of-molecules-109145">everything you can touch</a> is made up of lots of tiny molecules, which are themselves made up of even smaller parts called atoms. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-is-everything-really-made-of-molecules-109145">Curious Kids: is everything really made of molecules?</a>
</strong>
</em>
</p>
<hr>
<p>Water is also made of molecules. But during a ripple, the water molecules don’t move away from the rock, as you might expect. They actually move up and down. When they move up, they drag the other molecules next to them up – then they move down, dragging the molecules next to them down too. </p>
<p>That’s what creates the peaks and troughs you see on the surface of the water. And that’s how the ripple travels away from your rock – a bit like a human wave around a stadium. </p>
<p><div data-react-class="InstagramEmbed" data-react-props="{"url":"https://www.instagram.com/p/BsCBWE5lHr4","accessToken":"127105130696839|b4b75090c9688d81dfd245afe6052f20"}"></div></p>
<p>Dragging neighbouring water molecules up and down is hard work, and slowly uses up energy, so the ripples get smaller as they get further away. Eventually, the ripples use up all the energy from the rock and the splash, and shrink until we can no longer see them. </p>
<h2>Rippling out</h2>
<p>Ripples often spread out in circles, but this isn’t the only possibility. If you throw a stick into the water it will create straight ripples on the sides, and round ripples near the ends. So your rock probably made circular ripples because the rock itself was quite round. </p>
<p>But something else is happening too: different waves move at different speeds. Waves with a lot of energy move more quickly. For example, really big tidal waves, or tsunamis, race across the ocean <a href="https://www.nationalgeographic.com/news/2005/1/tsunamis-facts-about-killer-waves/">as fast as a plane flies</a> (up to 800 kilometres per hour). </p>
<p>When you throw a stick into the water, the ripples from the middle of the stick eventually catch up with the ripples from the ends, because of the different ways they spread out. So far away from the stick, the ripples are round … just like they were for your rock.</p>
<hr>
<p><em>Children can have their own questions answered by experts – just send them in to <a href="https://theconversation.com/au/topics/curious-kids-36782">Curious Kids</a>, along with the child’s first name, age and town or city. You can:</em></p>
<ul>
<li><em>email <a href="mailto:curiouskids@theconversation.com">curiouskids@theconversation.com</a></em></li>
<li><em>tweet us <a href="https://twitter.com/ConversationUK">@ConversationUK</a> with #curiouskids</em></li>
<li><em>DM us on Instagram <a href="https://www.instagram.com/theconversationdotcom/">@theconversationdotcom</a></em></li>
</ul>
<p><em>Here are some more <a href="https://theconversation.com/topics/curious-kids-36782?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=CuriousKidsUK">Curious Kids</a> articles, written by academic experts:</em></p>
<ul>
<li><p><em><a href="https://theconversation.com/curious-kids-how-does-our-brain-send-signals-to-our-body-124950?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=CuriousKidsUK">How does our brain send signals to our body? – Aarav, aged nine, Mumbai, India.</a></em></p></li>
<li><p><em><a href="https://theconversation.com/curious-kids-how-can-we-see-what-we-are-imagining-as-well-as-whats-in-front-of-us-124944?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=CuriousKidsUK">How can we see what we are imagining but still see what’s in front of us? – Malala Yousafzai class, Globe Primary School, London, UK.</a></em></p></li>
<li><p><em><a href="https://theconversation.com/curious-kids-why-is-the-sea-salty-124743?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=CuriousKidsUK">Why is the sea salty? – Torben, aged nine, Sussex, UK.</a></em></p></li>
</ul><img src="https://counter.theconversation.com/content/120308/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Simon Cox does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The simple experiment of throwing a rock into water actually reveals some fundamental rules of physics.Simon Cox, Professor of Mathematics, Aberystwyth UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/782292017-06-21T10:31:12Z2017-06-21T10:31:12ZReverse engineering mysterious 500-million-year-old fossils that confound our tree of life<figure><img src="https://images.theconversation.com/files/174773/original/file-20170620-5990-1lz2lmr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Here's the fossil... what can you tell about how this animal lived?</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Parvancorina_minchami_-_MUSE.jpg">Matteo De Stefano/MUSE-Science Museum</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Paleontologists like us are used to working with fossils that would seem bizarre to many biologists accustomed to living creatures. And as we go farther back in Earth’s history, the fossils start to look even weirder. They lack tails, legs, skeletons, eyes…any characteristics that would help us understand where these organisms fit in the tree of life. Under these circumstances, the science of paleontology becomes significantly harder.</p>
<p>Nowhere is this issue more apparent than in <a href="https://doi.org/10.1080/00241160500409223">the Ediacaran period</a>, which lasted from 635 million to 541 million years ago. A peculiar and entirely soft-bodied suite of fossils from this era are collectively referred to <a href="https://doi.org/10.1146/annurev.earth.33.092203.122519">as the Ediacara biota</a>. Despite <a href="http://www.biodiversitylibrary.org/page/41347851#page/235/mode/1up">nearly 70 years of careful study</a>, paleontologists have yet to identify key features among them that would allow us to understand how these organisms are related to modern animals. The forms evident among Ediacaran organisms are, for the most part, truly unique – and we are no closer to understanding their place in evolutionary history.</p>
<p>Rather than looking for characteristics that would allow us to shoehorn some of these organisms into known animal groups, <a href="https://doi.org/10.1098/rsbl.2017.0033">we’ve taken a different approach</a>. It relies on a technique called computational fluid dynamics that lets us reverse engineer how these organisms lived in their ocean environment.</p>
<h2>Mystery fossils</h2>
<p>The Ediacaran period marks a pivotal interval in Earth’s history; at its start are the last of the so-called “<a href="https://theconversation.com/us/topics/snowball-earth-16060">Snowball Earth</a>” events – episodes lasting millions of years when the entire surface of our planet was covered in ice. It segues into the succeeding Cambrian geological period, which saw the first appearance of many of the animal groups we recognize in the present day. This is what’s commonly referred to as the <a href="https://theconversation.com/uk/topics/cambrian-explosion-7159">Cambrian explosion</a>.</p>
<p>When large, complex fossils were discovered in the Ediacaran, researchers naturally expected that many of them would represent early relatives of the same animal groups that had been recognized in the Cambrian. But these Ediacarans seem completely distinct from modern animals.</p>
<p>For instance, <a href="https://doi.org/10.1126/science.1099727">the rangeomorphs</a> were a collection of leaf- and mat-like organisms with a unique fractal architecture, constructed from a series of branching “frond” elements, each a few centimeters long, each of which is itself composed of smaller, identical frond elements.</p>
<p>Another – <em>Tribrachidium</em> – was a small hemispherical organism possessing three raised branches that meet at the top of the organism and which curved toward the margin in a counterclockwise direction.</p>
<p>So how do oddballs like these fit in with what came before and what came after? We just haven’t been able to place them on any evolutionary tree.</p>
<p>In order to better understand these organisms, paleontologists have been forced to adopt a different approach. We’ve abandoned all assumptions about what they might be related to, and instead tried to answer more fundamental questions. For instance, did they move? How did they feed? How did they reproduce? By answering these questions, we can begin to understand their biology and ecology, which in turn may provide hints as to how these organisms are related to other multicellular lifeforms. This is how we’ve begun to reverse engineer the Ediacara biota. </p>
<h2>Modeling fluid dynamics to reverse engineer fossils</h2>
<p>One of the most important techniques at our disposal is computational fluid dynamics (CFD), a method for virtually simulating fluid flows around objects using computers.</p>
<p>The rationale for using this approach lies in observing organisms in modern oceans. We know that many (if not all) animals living in shallow marine environments have evolved adaptations that allow them to interact with and manipulate currents, either to reduce drag and prevent them from being swept away (think limpets and barnacles), or to aid in feeding (think crinoids, sea anemones and gorgonian corals). So we can learn a lot about an organism’s biology and ecology by studying the way it behaves in moving fluids.</p>
<p>With modern species, researchers can study fluid flows around living animals. But for organisms that have been extinct for over half a billion years – such as the Ediacara biota – virtual simulations using CFD are the only approach.</p>
<p>Here’s how we do it. First, we obtain a digital 3-D model of a fossil and place it in a virtual flume tank. Then, we simulate water flowing over and around the digital fossil. Visualizing patterns of flow and recirculation around the organism allows us to test hypotheses about how the organism moved and fed. With something as mysterious and obscure as the Ediacara biota, these insights may bring us closer to understanding what they are.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/174776/original/file-20170620-32348-25ho6r.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/174776/original/file-20170620-32348-25ho6r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/174776/original/file-20170620-32348-25ho6r.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=251&fit=crop&dpr=1 600w, https://images.theconversation.com/files/174776/original/file-20170620-32348-25ho6r.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=251&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/174776/original/file-20170620-32348-25ho6r.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=251&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/174776/original/file-20170620-32348-25ho6r.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=315&fit=crop&dpr=1 754w, https://images.theconversation.com/files/174776/original/file-20170620-32348-25ho6r.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=315&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/174776/original/file-20170620-32348-25ho6r.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=315&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Schematic reconstructions of various <em>Parvancorina</em> species.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Parvancorina_species.png">Aleksey Nagovitsyn</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Our recent work with the enigmatic Ediacaran fossil <em>Parvancorina</em> is an example of this approach. <em>Parvancorina</em> is a simple-looking, shield-shaped organism typically 1-2 centimeters in length, with an anchor-like series of ridges on its top surface. Although it’s been interpreted in a variety of ways, most scientists have assumed that it was fixed on the seafloor – what we call sessile. No one has seen any limbs preserved with <em>Parvancorina</em> and it’s never been found in association with fossilized tracks or trails.</p>
<p>We decided to test this idea by building 3-D models of the two known <em>Parvancorina</em> species, and then using CFD to see how their unique surface structures affected patterns of fluid flow in different orientations. Our results showed that patterns of water flow around the model were dramatically different depending on the how it was oriented in the current.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/174769/original/file-20170620-32365-1mgzxxh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/174769/original/file-20170620-32365-1mgzxxh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/174769/original/file-20170620-32365-1mgzxxh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=380&fit=crop&dpr=1 600w, https://images.theconversation.com/files/174769/original/file-20170620-32365-1mgzxxh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=380&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/174769/original/file-20170620-32365-1mgzxxh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=380&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/174769/original/file-20170620-32365-1mgzxxh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=478&fit=crop&dpr=1 754w, https://images.theconversation.com/files/174769/original/file-20170620-32365-1mgzxxh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=478&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/174769/original/file-20170620-32365-1mgzxxh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=478&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Digital models of <em>Parvancorina</em>. Center and right columns: results of CFD analyses showing fluid flow around <em>Parvancorina</em> in different orientations. Flow direction is indicated by arrows, and velocity indicated by colors (in meters per second), with faster flow in red colors, and slow flow in blue colors.</span>
<span class="attribution"><span class="source">Darroch and Rahman</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Assuming <em>Parvancorina</em> was a suspension feeder, our results demonstrate that it would have been good at capturing the food in the ocean water only when it was oriented in a single specific direction. This is obviously bad news if you’re a sessile suspension feeder, <a href="https://doi.org/10.1126/sciadv.1500800">like some other members of the Ediacara biota</a>. If you rely on the current to carry water laden with nutrients and food particles to your mouth or feeding apparatus, you want that to happen no matter which way the current is flowing. If you’re stuck in one place and the current changes, you’ve got a problem if you can only gather food when it’s coming at you from one direction. Any other plausible style of feeding – for example, scavenging – would also imply these creatures had a mobile lifestyle.</p>
<p>We also used these simulations to calculate drag in different orientations. Although talking about front and back ends in <em>Parvancorina</em> is slightly problematic (because we can’t even tell whether it had anything resembling a head and tail), we usually think of the shield end as the front. We showed that the drag experienced by <em>Parvancorina</em> was typically lower when it was placed front-on to current, compared to when it was placed side-on. This is also bad news if you’re a sessile organism, because it leaves you open to being ripped from the sediment in strong currents.</p>
<p>The inference from these two observations is clear: <em>Parvancorina</em> was <a href="https://doi.org/10.1098/rsbl.2017.0033">better adapted to life as a mobile, rather than a sessile, organism</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/174779/original/file-20170620-32333-142po1q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/174779/original/file-20170620-32333-142po1q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/174779/original/file-20170620-32333-142po1q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/174779/original/file-20170620-32333-142po1q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/174779/original/file-20170620-32333-142po1q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/174779/original/file-20170620-32333-142po1q.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/174779/original/file-20170620-32333-142po1q.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/174779/original/file-20170620-32333-142po1q.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">No one knows what life was like in the seas of the Ediacaran period, but research like this can start to fill in some details.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/ideonexus/2237406519">Ryan Somma</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>New understanding of <em>Parvancorina</em> lifestyle</h2>
<p>This conclusion may sound like a minor footnote in the story of life on Earth. But we believe it has powerful implications for how we view the Ediacara biota as a whole.</p>
<p>First, so little is currently known about <em>Parvancorina</em> that any additional information is crucial. The knowledge that it was mobile will help us work out where this fossil fits in the tree of life.</p>
<p>Second, the inference that <em>Parvancorina</em> was mobile, but nonetheless left no trace of its movement, is important – it means that many other Ediacaran fossils that we’ve assumed were sessile may actually have been mobile as well. This may require us to reimagine Ediacaran ecosystems as much more dynamic and, by extension, much more complex than we previously thought.</p>
<p>Through using tools like computational fluid dynamics to reverse engineer the Ediacara biota, we’re getting closer to understanding what they represent, and how they lived and functioned 15 million years before the Cambrian explosion.</p><img src="https://counter.theconversation.com/content/78229/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Simon Darroch receives funding from the Paleontological Society and National Geographic. </span></em></p><p class="fine-print"><em><span>Imran Rahman receives funding from the Oxford University Museum of Natural History. </span></em></p>With no identifiable body parts, it’s hard to know how these fossilized creatures lived. A new approach models how the ocean’s water would interact with their unique shapes – hinting at their lifestyle.Simon Darroch, Assistant Professor of Earth and Environmental Sciences, Vanderbilt UniversityImran Rahman, Junior Research Fellow, University of OxfordLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/766962017-04-27T01:50:57Z2017-04-27T01:50:57ZPhysics of poo: Why it takes you and an elephant the same amount of time<figure><img src="https://images.theconversation.com/files/166915/original/file-20170427-1835-114rk68.jpg?ixlib=rb-1.1.0&rect=1020%2C7%2C3968%2C2778&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Defecation duration is surprisingly similar throughout the mammal world.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/almost-done-pooping-african-bush-elephant-417852799">Elephant image via www.shutterstock.com.</a></span></figcaption></figure><p>The ancient Chinese practiced <a href="http://newsfeed.time.com/2013/11/21/the-absolute-grossest-way-to-have-your-fortune-read/">copromancy</a>, the diagnosis of health based on the shape, size and texture of feces. So did the Egyptians, the Greeks and <a href="http://www.randomhousebooks.com/books/100821/">nearly every ancient culture</a>. Even today, your doctor may ask when you last had a bowel movement and to describe it in exquisite detail.</p>
<p>Sure, it’s uncomfortable to talk about. But that’s where science comes in, because what we don’t like to discuss can still cause harm. <a href="http://www.mayoclinic.org/diseases-conditions/irritable-bowel-syndrome/basics/definition/con-20024578">Irritable bowel syndrome</a>, <a href="http://www.mayoclinic.org/diseases-conditions/inflammatory-bowel-disease/basics/definition/con-20034908">inflammatory bowel disease</a>, <a href="http://www.biomerieux-diagnostics.com/gastrointestinal-infections">gastrointestinal infections</a> and other poop-related ailments cost Americans <a href="http://www.ajmc.com/journals/supplement/2016/importance_of_selecting_appropriate_therapy_inflammatory_bowel_disease_managed_care_environment/importance_of_selecting_appropriate_therapy_inflammatory_bowel_disease_managed_care_environment_report_economic_implications_ibd">billions of dollars annually</a>. </p>
<p>But trying to stem these problems was not our main motivation for trying to figure out <a href="https://doi.org/10.1039/C6SM02795D">some of the physics of defecation</a>. It was something else, much more sinister. </p>
<h2>From personal observation, into the lab</h2>
<p>When parenthood hits, it hits hard. <a href="http://www.me.gatech.edu/faculty/hu">One of us</a> is a working dad who survived by learning a new set of skills, one of which was fecal analysis. Years of diaper changes and then potty training turned me from a poo-analysis novice to a wizened connoisseur. My life passes by in a series of images: hard feces pellets like peas to long feces like a smooth snake to a puddle of brown water.</p>
<p>Unlike the ancients, we didn’t believe that we could predict the future from children’s stool. But we did think it was worth trying to understand where all these shapes come from. <a href="http://www.hu.gatech.edu">Having a laboratory</a> to answer questions about the everyday world is one of the distinct pleasures of being a scientist.</p>
<p>As <a href="http://pyang.gatech.edu/">fluid dynamicists</a>, we joined forces with colorectal surgeon <a href="https://www.uab.edu/medicine/surgery/gastrointestinal/faculty/chu">Daniel Chu</a>, and two stalwart undergraduates, Candice Kaminski and Morgan LaMarca, who filmed defecation and hand-picked feces from 34 mammalian species at <a href="https://zooatlanta.org/">Zoo Atlanta</a> in order to measure their density and viscosity.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/AG-jIfslXII?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Raw footage of an elephant at the Atlanta Zoo.</span></figcaption>
</figure>
<p>We learned that most elephants and other herbivores create “floaters” while most tigers and other carnivores create “sinkers.” Inadvertently, we also ranked feces from most to least smelly, starting with tiger and rhino and going all the way to panda. The zoo’s variety of animals provided us with a range of fecal sizes and shapes that served as independent pieces of evidence to validate our mathematical model of the duration of defecation.</p>
<p>We also placed the feces in a device called a “rheometer,” a precision blender that can measure the properties of liquid-like and solid-like materials such as chocolate and shampoo. Our lab shares two rheometers with Georgia Tech physicist <a href="https://www.physics.gatech.edu/user/alberto-fernandez-nieves">Alberto Fernandez-Nieves</a>. We have since categorized the rheometers as the “clean rheometer” and the “David Hu rheometer” – which has seen its fair share of <a href="https://theconversation.com/the-frog-tongue-is-a-high-speed-adhesive-72064">frog saliva</a>, mucus and feces.</p>
<h2>The secret to the speed</h2>
<p>What else did we learn? Bigger animals have longer feces. And bigger animals also defecate at higher speed. For instance, an elephant defecates at a speed of six centimeters per second, nearly six times as fast as a dog. The speed of defecation for humans is in between: two centimeters per second.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/166887/original/file-20170426-2828-1hvnex8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/166887/original/file-20170426-2828-1hvnex8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/166887/original/file-20170426-2828-1hvnex8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=281&fit=crop&dpr=1 600w, https://images.theconversation.com/files/166887/original/file-20170426-2828-1hvnex8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=281&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/166887/original/file-20170426-2828-1hvnex8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=281&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/166887/original/file-20170426-2828-1hvnex8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=354&fit=crop&dpr=1 754w, https://images.theconversation.com/files/166887/original/file-20170426-2828-1hvnex8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=354&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/166887/original/file-20170426-2828-1hvnex8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=354&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The relationship between body mass M and defecation time. Symbols represent experimental measurements; dashed line represents best fit to the data; solid line represents the theoretical prediction.</span>
<span class="attribution"><span class="source">Yang et al, DOI: 10.1039/C6SM02795D</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Together, this meant that defecation duration is constant across many animal species – around 12 seconds (plus or minus 7 seconds) – even though the volume varies greatly. Assuming a bell curve distribution, 66 percent of animals take between 5 and 19 seconds to defecate. It’s a surprisingly small range, given that elephant feces have a volume of 20 liters, nearly a thousand times more than a dog’s, at 10 milliliters. How can big animals defecate at such high speed?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/166889/original/file-20170426-2838-1bwv58p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/166889/original/file-20170426-2838-1bwv58p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/166889/original/file-20170426-2838-1bwv58p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=295&fit=crop&dpr=1 600w, https://images.theconversation.com/files/166889/original/file-20170426-2838-1bwv58p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=295&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/166889/original/file-20170426-2838-1bwv58p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=295&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/166889/original/file-20170426-2838-1bwv58p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=371&fit=crop&dpr=1 754w, https://images.theconversation.com/files/166889/original/file-20170426-2838-1bwv58p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=371&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/166889/original/file-20170426-2838-1bwv58p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=371&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Mucus on the surface of rat feces shines at t = 0 and evaporates in less than 30 seconds.</span>
<span class="attribution"><span class="source">Yang et al, DOI: 10.1039/C6SM02795D</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The answer, we found, was in the properties of an ultra-thin layer of <a href="https://doi.org/10.1097/MPG.0b013e3181a117ea">mucus lining the walls of the large intestine</a>. The mucus layer is as thin as human hair, so thin that we could measure it only by weighing feces as the mucus evaporated. Despite being thin, the mucus is very slippery, more than 100 times less viscous than feces.</p>
<p>During defecation, feces moves like a solid plug. Therefore, in ideal conditions, the combined length and diameter of feces is simply determined by the shape of one’s rectum and large intestine. One of the big findings of our study was that feces extend halfway up the length of the colon from the rectum.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/7OPT29L5YoE?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Pandas poo, too.</span></figcaption>
</figure>
<h2>A unified theory of pooping</h2>
<p>Putting the length of feces together with the properties of mucus, we now have a cohesive physics story for how defecation happens. Bigger animals have longer feces, but also thicker mucus, enabling them to achieve high speeds with the same pressure. Without this mucus layer, defecation might not be possible. Alterations in mucus can contribute to several ailments, including <a href="http://www.webmd.com/digestive-disorders/features/chronic-constipation-facts-vs-myths#1">chronic constipation</a> and even infections by bacteria such as <a href="https://doi.org/10.1152/ajpgi.00091.2014">C. difficile</a> in the gastrointestinal tract.</p>
<p>Beyond simply following our scientific curiosity, our measurements of feces have also had some practical applications. Our defecation data helped us design an adult diaper for astronauts. Astronauts want to stay in space suits for seven days, but are limited by their diapers.
Taking advantage of the viscosity of feces, we designed a diaper that segregates the feces away from direct contact with skin. It was a <a href="https://herox.com/SpacePoop/update/1277?kme=challenge_post_email&km_challenge_post_email=1277">semifinalist</a> in the <a href="https://herox.com/SpacePoop">NASA Space Poop Challenge</a> in 2017.</p>
<p>It just shows that physics and mathematics can be used everywhere, even in your toilet bowl.</p><img src="https://counter.theconversation.com/content/76696/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>New parenthood got our fluid dynamics experts thinking about what ends up in the diaper. They headed to the zoo and the lab to come up with a cohesive physics story for how defecation works.David Hu, Associate Professor of Mechanical Engineering and Biology, Adjunct Associate Professor of Physics, Georgia Institute of TechnologyPatricia Yang, Ph.D. Student in Mechanical Engineering, Georgia Institute of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/748582017-03-22T10:33:40Z2017-03-22T10:33:40ZWe may just have solved the great mystery of why drops splash<figure><img src="https://images.theconversation.com/files/161848/original/image-20170321-5391-lhfxgo.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/water-splashwater-splash-isolated-on-backgroundwater-573666874?src=sv-vkrkfiyUnpA-vYq4Oog-1-16">CK Foto/Shutterstock</a></span></figcaption></figure><p>From the raindrops that soak you on your way to work to the drops of coffee that inevitably end up on your white shirt when you arrive, you’d be forgiven for thinking of drops as a mere nuisance. </p>
<p>But beneath a mundane facade, droplets exhibit natural beauty and conceal complex physics that scientists have been trying to figure out for decades. Recently, I have contributed to this field by <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.114502">working on a new theory</a> explaining what happens to the critical thin layer of air between a drop of water and a surface to cause a splash. </p>
<p>At just a few thousandths of a second, the lifetime of a splashing drop is too rapid for us to see. It took pioneering advances in high-speed imaging to capture these events – the most iconic being <a href="http://edgerton-digital-collections.org/?s=hee-nc-57001#hee-nc-57001">Edgerton’s Milk Drop Coronet in 1957</a>. These pictures simultaneously captured the public’s imagination with their aesthetic nature while intriguing physicists with their surprising complexity. The most obvious question is why, and when, do drops splash?</p>
<p>Nowadays, cameras can take over a million frames per second and resolve the fine details of a splash. However, these advances have raised as many questions as they have answered. Most importantly, <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.94.184505">remarkable observations</a>, coming from the <a href="http://nagelgroup.uchicago.edu/Nagel-Group/researchdir/splashing.html">NagelLab</a> in 2005, showed that the air surrounding the drop plays a critical role. By reducing the air pressure, one can prevent a splash (see second video). In fact, drops which splash at the bottom of Mount Everest may not do so at the top, where the air pressure is lower.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/NkLm_RTR_wc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Ethanol drop at normal pressure splashes.</span></figcaption>
</figure>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/_u-eNgrQOf8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Ethanol drop at low pressure doesn’t splash.</span></figcaption>
</figure>
<p>The discoveries created an explosion of experimental work aimed at uncovering the curious details of the air’s role. <a href="http://annualreviews.org/doi/abs/10.1146/annurev-fluid-122414-034401">New experimental methods</a> revealed incredible dynamics: millimetre-sized liquid drops are controlled by the behaviour of microscopic air films that are 1,000 times smaller. </p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/161891/original/image-20170321-5384-pbvbqd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/161891/original/image-20170321-5384-pbvbqd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161891/original/image-20170321-5384-pbvbqd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161891/original/image-20170321-5384-pbvbqd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161891/original/image-20170321-5384-pbvbqd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161891/original/image-20170321-5384-pbvbqd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161891/original/image-20170321-5384-pbvbqd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Coffee splash pattern on the right next to ring from mug.</span>
<span class="attribution"><span class="source">Roger Karlsson/Flickr</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Notably, after a liquid drop contacts a solid it can be prevented from spreading across it by a microscopically thin layer of air that it can’t push aside. The sizes involved are equivalent to a one-centimetre layer of air stopping a tsunami wave spreading across a beach. When this occurs, a sheet of liquid can fly away from the main drop and break into smaller droplets – <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.024507">so that a splash is generated</a>.
From a coffee stain all we can see is the outcome of this event – a pool of liquid (the drop) surrounded by a ring of smaller drops (the splash).</p>
<h2>Major breakthrough</h2>
<p>Experimental analyses have produced incredibly detailed observations of drops splashing. But they do not establish <em>why</em> the drops splash, which means we don’t understand the underlying physics. Remarkably, for such a seemingly innocuous problem the <a href="https://www.grc.nasa.gov/www/k-12/airplane/nseqs.html">classical theory of fluids</a> – used to forecast weather, design ships and predict blood flow – is inadequate. This is because the air layer’s height becomes comparable to the distance air molecules travel between collisions. So for this specific problem we need to feed in microscopic details that the classical theory simply doesn’t account for.</p>
<figure> <img src="http://www2.warwick.ac.uk/newsandevents/pressreleases/why_water_splashes/splash-animation_final.gif"><figcaption>How a microscopic layer of air affects water droplets.</figcaption></figure>
<p>The air’s behaviour can only be captured by a theory originally developed for violent aerodynamic gas flows – such as for space shuttles entering the Earth’s atmosphere – namely the <a href="https://en.wikipedia.org/wiki/Kinetic_theory_of_gases">kinetic theory of gases</a>. My new article, <a href="http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.114502">published in Physical Review Letters</a>, is the first to use kinetic theory to understand how the air film behaves as it is displaced by a liquid spreading over a solid. </p>
<p>The article establishes criteria for the maximum speed at which a liquid can stably spread over a solid. It was already known that for a splash to be produced, this critical speed must be exceeded. If the speed is lower than that, the drop spreads smoothly instead. Notably, the new theory explains why reducing the air pressure can suppress splashing: in this case, air escapes more easily from the layer and provides less resistance to the liquid drop. This is the missing piece of a jigsaw to which <a href="http://annualreviews.org/doi/abs/10.1146/annurev-fluid-122414-034401">numerous important scientific contributions</a> have been made since the experimental discoveries of 2005. </p>
<h2>Important applications</h2>
<p>While being of fundamental scientific interest, an understanding of the conditions that cause splashing can be exploited – leading to potential breakthroughs in a number of practical fields. </p>
<p>One example is <a href="https://theconversation.com/explainer-what-is-3d-printing-and-whats-it-for-9456">3D printing</a> where liquid drops form the building blocks of tailor-made products such as hearing aids. Here, stopping splashing is key to making products of the desired quality. Another important area is forensic science, where <a href="http://www.forensicsciencesimplified.org/blood/">blood-stain-pattern analysis</a> relies on splash characteristics to provide insight into where the blood came from – yielding vital information in a criminal investigation.</p>
<p>Most promisingly, the new theory will have applications to a wide range of related flows where microscopic layers of air appear. For example, in climate science it will enable us to understand how water drops collide during the formation of clouds and to estimate the quantity of gas being dragged into our oceans by rainfall.</p>
<p>Do keep this in mind the next time you splatter coffee drops across your desk. Take a moment to admire the pattern and appreciate the underlying complexity before cursing and heading for your “mopper upper” of choice.</p><img src="https://counter.theconversation.com/content/74858/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>James Sprittles receives funding from the Leverhulme Trust and the Engineering and Physical Sciences Research Council (EP/N016602/1).
Dr Sprittles is also a part of Micro & Nano Flows for Engineering, a research partnership between the Universities of Warwick and Edinburgh, and Daresbury Laboratory.</span></em></p>Mathematicians make a splash with new theory that could lead to breakthroughs in 3D printing, climate science and forensics.James Sprittles, Assistant Professor in Mathematics, University of WarwickLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/720642017-02-01T00:02:26Z2017-02-01T00:02:26ZThe frog tongue is a high-speed adhesive<figure><img src="https://images.theconversation.com/files/154678/original/image-20170130-7894-20l6t5.jpg?ixlib=rb-1.1.0&rect=5%2C410%2C3384%2C2273&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Gotcha, five times faster than the blink of an eye. </span> <span class="attribution"><span class="source">Candler Hobbs/Georgia Tech</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>How does one get stuck studying frog tongues? Our study into the sticky, slimy world of frogs all began with a <a href="https://www.youtube.com/watch?v=LbNl3J8HXw4">humorous video</a> of a real African bullfrog lunging at fake insects in a mobile game. This frog was clearly an expert at gaming; the speed and accuracy of its tongue could rival the thumbs of texting teenagers.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/LbNl3J8HXw4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The frog that was the genesis of the authors’ tongue research.</span></figcaption>
</figure>
<p>Further YouTube research yielded amazing videos of frogs eating <a href="https://www.youtube.com/watch?v=6AILF4Spwpo">mice</a>, <a href="https://www.youtube.com/watch?v=b-K0KcbUWmI">tarantulas</a> and even other <a href="https://www.youtube.com/watch?v=2kwrWl8zWdg">frogs</a>. </p>
<p>The versatile frog tongue can grab wet, hairy and slippery surfaces with equal ease. It does a lot better than our engineered adhesives – not even household tapes can firmly stick to wet or dusty surfaces. What makes this tongue even more impressive is its speed: Over 4,000 species of frog and toad <a href="http://psycnet.apa.org/psycinfo/1990-97523-000">snag prey faster than a human can blink</a>. What makes the frog tongue so uniquely sticky? <a href="http://dx.doi.org/10.1098/rsif.2016.0764">Our group aimed to find out</a>.</p>
<h2>Baseline frog tongue knowledge</h2>
<p>Early modern scientific attention to frog tongues came in 1849, when biologist Augustus Waller published the <a href="http://dx.doi.org/10.1098/rstl.1849.0010">first documented frog tongue study</a> on nerves and papillae – the surface microstructures found on the tongue. Waller was fascinated with the soft, sticky nature of the frog tongue and what he called:</p>
<blockquote>
<p>“the peculiar advantages possessed by the tongue of the living frog…the extreme elasticity and transparency of this organ induced me to submit it to the microscope.”</p>
</blockquote>
<p>Fast-forward 165 years, when biomechanics researchers Kleinteich and Gorb were the first to <a href="http://dx.doi.org/10.1038/srep05225">measure tongue forces in the horned frog</a> <em>Ceratophrys cranwelli</em>. They found in 2014 that frog adhesion forces can reach up to 1.4 times the body weight. That means the sticky frog tongue is strong enough to lift nearly twice its own weight. They postulated that the <a href="http://dx.doi.org/10.1098/rsos.150333">tongue acts like sticky tape or a pressure-sensitive adhesive</a> – a permanently tacky surface that adheres to substrates under light pressure.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/155018/original/image-20170131-3244-7r5phb.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Frog tongue holding up a petri dish just with its stickiness.</span>
<span class="attribution"><span class="source">Alexis Noel/Georgia Tech</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>To begin our own study on sticky frog tongues, we filmed various frogs and toads eating insects using high-speed videography. We found that the frog’s tongue is able to capture an insect in under 0.07 seconds, five times faster than a human eye blink. In addition, insect acceleration toward the frog’s mouth during capture can reach 12 times the acceleration of gravity. For comparison, astronauts normally experience around three times the acceleration of gravity during a rocket launch.</p>
<h2>On to the materials testing</h2>
<p>Thoroughly intrigued, we wanted to understand how the sticky tongue holds onto prey so well at high accelerations. We first had to gather some frog tongues. Here at Georgia Tech, we tracked down an on-campus biology dissection class, who used <a href="https://en.wikipedia.org/wiki/Northern_leopard_frog">northern leopard frogs</a> on a regular basis.</p>
<p>The plan was this: Poke the tongue tissue to determine softness, and spin the frog saliva between two plates to determine viscosity. Softness and viscosity are common metrics for comparing solid and fluid materials, respectively. Softness describes tongue deformation when a stretching force is applied, and viscosity describes saliva’s resistance to movement. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1067&fit=crop&dpr=1 600w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1067&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1067&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1340&fit=crop&dpr=1 754w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1340&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/155021/original/image-20170131-3269-v89zmh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1340&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">When the indentation head pulls away from the tongue, it adheres and stretches.</span>
<span class="attribution"><span class="source">Alexis Noel/Georgia Tech</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Determining the softness of frog tongue tissue was no easy task. We had to create our own indentation tools since the tongue softness was beyond the capabilities of the traditional materials-testing equipment on campus. We decided to use an indentation machine, which pokes biological materials and measures forces. The force-displacement relationship can then describe softness based on the indentation head shape, such as a cylinder or sphere.</p>
<p>However, typical heads for indentation machines can cost US$500 or more. Not wanting to spend the money or wait on shipping, we decided to make our own spherical and flat-head indenters from stainless steel earrings. After our tests, we found frog tongues are about as soft as brain tissue and 10 times softer than the human tongue. Yes, we tested brain and human tongue tissue (post mortem) in the lab for comparison.</p>
<p>For testing saliva properties, we ran into a problem: The machine that would spin frog saliva required about one-fifth of a teaspoon of fluid to run the test. Sounds small, but not in the context of collecting frog spit. Amphibians are unique in that they <a href="http://dx.doi.org/10.1111/j.1463-6395.1969.tb00527.x">secrete saliva through glands located on their tongue</a>. So, one night we spent a few hours scraping 15 dead frog tongues to get a saliva sample large enough for the testing equipment.</p>
<p>How do you get saliva off a frog tongue? Easy. First, you pull the tongue out of the mouth. Second, you rub the tongue on a plastic sheet until a (tiny) saliva globule is formed. Globules form due to the long-chain mucus proteins that exist in the frog saliva, much like human saliva; these <a href="http://dx.doi.org/10.1007/BF00305337">proteins tangle like pasta when swirled</a>. Then you quickly grab the globule using tweezers and place it in an airtight container to reduce evaporation.</p>
<p>After testing, we were surprised to find that the saliva is a two-phase viscoelastic fluid. The two phases are dependent on how quickly the saliva is sheared, when resting between parallel plates. At low shear rates, the saliva is very thick and viscous; at high shear rates, the frog saliva becomes thin and liquidy. This is similar to paint, which is easily spread by a brush, yet remains firmly adhered on the wall. Its these two phases that give the saliva its reversibility in prey capture, for adhering and releasing an insect.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/26xFAMpG2R8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A South American horned frog demonstrates capturing a cricket in this slow motion video.</span></figcaption>
</figure>
<h2>To catch a cricket</h2>
<p>How does soft tissue and a two-phase saliva help the frog tongue stick to an insect? Let’s walk through a prey-capture scenario, which begins with a frog tongue zooming out of the mouth and slamming into an insect.</p>
<p>During this impact phase, the tongue deforms and wraps around the insect, increasing contact area. The saliva becomes liquidy, penetrating the insect cracks. As the frog pulls its tongue back into the mouth, the tissue stretches like a spring, reducing forces on the insect (similar to how a bungee cord reduces forces on your ankle). The saliva returns to its thick, viscous state, maintaining high grip on the insect. Once the insect is inside the mouth, the eyeballs push the insect down the throat, causing the saliva to once again become thin and liquidy. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/fwThZXXXdTc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Frogs’ eyeballs actually help them swallow their food by physically pushing what’s in the mouth back into the throat.</span></figcaption>
</figure>
<p>It’s possible that <a href="http://dx.doi.org/10.1098/rsif.2016.0764">untangling the adhesion secrets of frog tongues</a> could have future applications for things like high-speed adhesive mechanisms for conveyor belts, or fast grabbing mechanisms in soft robotics.</p>
<p>Most importantly, this work provides valuable insight into the biology and function of amphibians – 40 percent of which are in <a href="http://www.iucnredlist.org">catastrophic decline or already extinct</a>. Working with conservation organization <a href="http://www.amphibianfoundation.org">The Amphibian Foundation</a>, we had access to live and preserved species of frog. The results of our research provide us with a greater understanding of this imperiled group. The knowledge gathered on unique functions of frog and toad species can inform conservation decisions for managing populations in dynamic and declining ecosystems.</p>
<p>While it’s not easy being green, a frog may find comfort in the fact that its tongue is one amazing adhesive.</p>
<hr>
<p><em>Mark Mandica of <a href="http://www.amphibianfoundation.org">The Amphibian Foundation</a> collaborated on the <a href="http://dx.doi.org/10.1098/rsif.2016.0764">research published in Journal of the Royal Society Interface</a> and coauthored this article.</em></p><img src="https://counter.theconversation.com/content/72064/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>This material is based upon work supported by the National Science Foundation Graduate Research Fellowship (DGE-1148903).</span></em></p><p class="fine-print"><em><span>David Hu does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>How do a frog’s tongue and saliva work together to be sticky enough to lift 1.4 times the animal’s body weight? Painstaking lab work found their spit switches between two distinct phases to nab prey.Alexis Noel, PhD Student in Biomechanics, Georgia Institute of TechnologyDavid Hu, Associate Professor of Mechanical Engineering and Biology, Adjunct Associate Professor of Physics, Georgia Institute of TechnologyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/568772016-03-29T15:01:14Z2016-03-29T15:01:14ZWhat can fish mouths teach us about engineering clog-free filters?<figure><img src="https://images.theconversation.com/files/116536/original/image-20160328-17844-f3fapm.jpg?ixlib=rb-1.1.0&rect=1046%2C404%2C3241%2C2516&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Filter-feeding fish have had 150 million years to improve filtration.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/usfwsmtnprairie/9546645557/">Rob Holm / USFWS</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Filter-feeding fish accomplish a feat that human technologies cannot: species including goldfish, menhaden and basking sharks filter tiny algal cells or shrimp-like prey from huge volumes of water without clogging their oral filters.</p>
<p>Since fish have been filtering particles for more than 150 million years longer than human beings, we suspected fish may have evolved filter designs that use unknown processes to remain unclogged. So we decided to investigate.</p>
<p>Our research, <a href="http://nature.com/articles/doi:10.1038/ncomms11092">recently published in <em>Nature Communications</em></a>, combines approaches from biomechanics, medicine and ecology to explore how these fish retain and transport prey inside their mouths. Our goal is to provide ideas and data that could improve aquaculture, conservation and industrial filtration.</p>
<h2>Crossflow filtration works for fish and industry</h2>
<p>Until 15 years ago, we thought that most filter-feeding fish used oral structures called gill rakers in the same way that we use coffee filters or spaghetti strainers. These so-called dead-end sieves force water to pass straight through the pores of the mesh. But dead-end sieves always clog as particles accumulate over time to cover the filter surface.</p>
<p>The water flows right through a colander and leaves the spaghetti trapped on the mesh, but a fish needs to move the food from the gill raker filter to the back of its mouth for swallowing. Dead-end sieves would cause problems for fish, since their gill rakers would clog and fish don’t have a tongue to move food particles off the gill rakers. So we knew they must be using some other filtering technique.</p>
<p>By putting a biomedical endoscope inside the mouths of feeding fish, <a href="http://doi.org/10.1038/35086574">colleagues and I discovered in 2001</a> that several common fish species use crossflow filtration instead of trapping particles directly on a dead-end sieve.</p>
<p>During crossflow filtration, small secondary streams of fluid pass through each filter pore – perpendicular to the filter surface, like in dead-end filtration. But the main stream of fluid – the “crossflow” – is directed to travel across (parallel to) the filter surface, lifting particles off the filter and preventing the pores from clogging with particles.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/116557/original/image-20160329-17835-zbfkb9.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/116557/original/image-20160329-17835-zbfkb9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/116557/original/image-20160329-17835-zbfkb9.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=170&fit=crop&dpr=1 600w, https://images.theconversation.com/files/116557/original/image-20160329-17835-zbfkb9.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=170&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/116557/original/image-20160329-17835-zbfkb9.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=170&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/116557/original/image-20160329-17835-zbfkb9.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=213&fit=crop&dpr=1 754w, https://images.theconversation.com/files/116557/original/image-20160329-17835-zbfkb9.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=213&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/116557/original/image-20160329-17835-zbfkb9.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=213&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A tilapia illustrating the current model of crossflow filtration, from Sanderson et al., doi: 10.1038/ncomms11092. The mainstream flow (MF) enters from the right and passes across the gill rakers (GR) that are attached to the branchial arches (BA). The mainstream flow carries concentrated particles to the back of the mouth for swallowing. The smaller secondary flows (the filtrate, Fi) pass through the pores of the gill raker filter.</span>
<span class="attribution"><span class="source">Virginia Greene, virginiagreeneillustration.com</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>Through the endoscope, we could see that the main flow of water heading toward the back of the mouth was transporting concentrated particles parallel to the gill raker filter. Less forceful streams of particle-free water exited between the gill rakers. All of these fluid dynamics are caused by the interaction of the water with the physical structures in the fish’s mouth.</p>
<p>We hadn’t expected to see crossflow filtration in fish, though this mechanism had been independently developed by industry a few decades earlier. Crossflow filtration avoids clogging and is often used to filter wastewater, pharmaceuticals, dairy foods and beverages such as beer and fruit juices.</p>
<p>Unfortunately, even <a href="https://en.wikipedia.org/wiki/Membrane_fouling">industrial crossflow filters still clog eventually</a>. Over time, as water exits through the filter pores, it deposits some particles on the filter. The filters must then be backflushed or cleaned with chemicals, causing a major operating expense.</p>
<p>So we turned again to fish, to see whether millions of years of evolution might have come up with unique crossflow filter designs.</p>
<h2>Biomimetic designs from fish mouths</h2>
<p>We started our study by examining basic structures inside fish mouths, familiar to fishermen and aquarium hobbyists. Fish gill rakers – the “feeding filters” – are attached to the branchial arches. These arches are bone or cartilage “ribs” inside the mouth that also support the bright red gills for gas exchange. The arches are typically positioned one after another from the front of the mouth back toward the esophagus, where food is swallowed. Scientists hadn’t previously considered the effects these branchial arches could have on patterns of water flow.</p>
<p>For our latest research, we made our own filters by using computer-aided design (CAD) software and 3D printing to create cone-shaped plastic models of fish mouths. We covered the branchial arch “ribs” with a fine nylon mesh.</p>
<p>We based our physical models on paddlefish and basking sharks because their branchial arches form a series of tall ribs that are separated by deep grooves. In our models, each rib served as a <a href="https://www.youtube.com/watch?v=JP94PR9vE7g">backward-facing step</a> that interacted with the crossflow of water traveling over the step.</p>
<p>Almost anywhere that water flows over a backward-facing step, a vortex is created automatically. For this reason, the closely-spaced tall ribs (“<em>d</em>-type ribs”) in these fish mouths aren’t often used by engineers because of the disruptive vortices that form continuously in the grooves between the ribs.</p>
<p>We designed many models with different versions of these backward-facing steps to test the effects of varying characteristics like height and distance between the steps. Interestingly, designs for some microfluidics devices that are used in labs for cell sorting have similar rib-like structures.</p>
<p>Both paddlefish and basking sharks are ram filter feeders that <a href="https://www.youtube.com/watch?v=7Y5c9l4Eev8">swim forward with a completely open mouth to capture prey</a>. To simulate this kind of feeding, my three undergraduate student coauthors, Erin Roberts, Jillian Lineburg and Hannah Brooks, and I conducted experiments in a flow tank. We submerged our stationary models in a constant stream of water inside the tank. The models “fed” on particles as we adjusted the speed of the water in the flow tank and added particles of different sizes, shapes and densities to the water.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/116558/original/image-20160329-17859-bmyuik.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/116558/original/image-20160329-17859-bmyuik.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/116558/original/image-20160329-17859-bmyuik.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=183&fit=crop&dpr=1 600w, https://images.theconversation.com/files/116558/original/image-20160329-17859-bmyuik.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=183&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/116558/original/image-20160329-17859-bmyuik.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=183&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/116558/original/image-20160329-17859-bmyuik.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=231&fit=crop&dpr=1 754w, https://images.theconversation.com/files/116558/original/image-20160329-17859-bmyuik.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=231&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/116558/original/image-20160329-17859-bmyuik.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=231&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A paddlefish illustrating the new vortical cross-step filtration model, from Sanderson et al., doi: 10.1038/ncomms11092. The mainstream flow (MF) enters from the right and interacts with the series of backward-facing steps that are formed by the branchial arches (BA), causing vortical flow (Vo). The vortex interacts with the gill rakers (GR) to concentrate particles for transport towards the back of the mouth to be swallowed.</span>
<span class="attribution"><span class="source">Virginia Greene, virginiagreeneillustration.com</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<h2>Unique vortical cross-step filtration in fish</h2>
<p>Like the spinning of a mini-tornado, water passed over the backward-facing steps inside our models and formed a distinct vortex in the groove between each pair of ribs. We designed accessory structures to control the movement of the vortices by creating regions of the model where the flow couldn’t escape easily. High shear rates around the vortices scoured particles off the mesh, preventing clogging.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/116532/original/image-20160328-17862-1jfs3q7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/116532/original/image-20160328-17862-1jfs3q7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/116532/original/image-20160328-17862-1jfs3q7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/116532/original/image-20160328-17862-1jfs3q7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/116532/original/image-20160328-17862-1jfs3q7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/116532/original/image-20160328-17862-1jfs3q7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/116532/original/image-20160328-17862-1jfs3q7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/116532/original/image-20160328-17862-1jfs3q7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Green dye helps visualize the vortices generated in model paddlefish and basking shark mouths.</span>
<span class="attribution"><span class="source">S. Laurie Sanderson</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>We manipulated the vortices to carry particles to the floor of the models, showing that fish could be using this highly adaptable filtration system like a “hydrodynamic tongue” to move particles inside their mouths.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/l03n9gVbkrc?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">We manipulated the vortices in our models to transport concentrated particles along the vortex axis, downstream from each backward-facing step. The vortices lifted particles from the mesh and carried them toward the floor of the model.</span></figcaption>
</figure>
<p>Small preserved paddlefish from an aquaculture company, placed in the flow tank in filter-feeding position, also formed vortices that concentrated particles inside the mouth. This suggests that we’ve correctly identified and modeled structures that are important for generating vortices inside real fish mouths.</p>
<p>This new filtration method, which we term “vortical cross-step filtration,” is effective even when the mesh is damaged or missing from a large portion of the models. Just like fish can continue to feed even when their gill rakers are still growing or are torn, our models can capture particles even when there are large holes in the mesh.</p>
<p>Although we’d identified vortices as a potential mechanism for fish filtration as early as 2001, data on particle capture by vortical flow in fish mouths haven’t been published previously.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/1T7MXCxbatM?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Rhodamine dye traces the path of a vortex that forms downstream from a backward-facing step. The step mimics a branchial arch inside a fish’s mouth.</span></figcaption>
</figure>
<h2>The future of cross-step filtration</h2>
<p>Our biomimetic models of paddlefish and basking shark mouths use novel arrangements of engineering structures that harness vortical flow to retain and transport tiny food particles. Cross-step filtration could also apply to filter-feeding ducks, baleen whales and the gill rakers of filter-feeding fish such as <a href="http://www.sciencefriday.com/videos/no-strain-no-gain-filter-feeding-mantas-2/">manta rays</a>.</p>
<p>Understanding these vortices in fish opens new research directions for engineering improved filters with less clogging, as well as the rapid separation of cells for biomedical tests.</p><img src="https://counter.theconversation.com/content/56877/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>S.Laurie Sanderson is the inventor on U.S. and international patent applications filed by the College of William & Mary (provisional filing 02/2014; nonprovisional filing 02/2015; 14/619,377; PCT/US15/15419). This research on filter-feeding fish was funded in part by the National Science Foundation (NSF Grant IBN-0131293 to S.L.S.).</span></em></p>Even the best engineered filters get clogged eventually. Fish mouths have evolved structures that create unique fluid dynamics patterns that solve that problem.S. Laurie Sanderson, Professor of Biology, William & MaryLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/461672015-08-18T23:03:16Z2015-08-18T23:03:16ZHummingbird tongues are tiny pumps that spring open to draw in nectar<figure><img src="https://images.theconversation.com/files/92319/original/image-20150818-12433-1defx1o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A juvenile male black-throated mango hummingbird (_Anthracothorax nigricollis_) extending his tongue after drinking nectar.</span> <span class="attribution"><a class="source" href="http://www.kristiinahurme.com/Hummingbirds.html">Kristiina Hurme</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>Hummingbirds live life at incomprehensible speeds. Their flight acrobatics are amazing, maneuvering <a href="http://doi.org/10.1098/rspb.2011.2238">more like insects than birds</a> as they flit around, flying upside down and even backwards. They’re a blur as they race between flowers. When they do pause to visit a flower momentarily, they’re licking 15 to 20 times a second to extract their nectar fuel.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/O_QyP810Riw?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Watch this speckled hummingbird (<em>Adelomyia melanogenys</em>) empty a flower in less than one second!</span></figcaption>
</figure>
<p>What makes them so intriguing to us is the result of this simple dietary choice: they drink nectar. Each flower doesn’t offer a lot, so to make a living off tiny amounts of nectar spread throughout the forest, hummingbirds are tiny, fast and feisty. </p>
<p>Feeding on nectar is hummingbirds’ defining characteristic, but until now scientists didn’t know the exact mechanics of how they do it. In our new study, we were able to slow them down on video to see <a href="http://rspb.royalsocietypublishing.org/content/282/1813/20151014">how they really drink nectar</a>. And what we found was quite different from the conventional wisdom since the 1800s.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/91956/original/image-20150815-5098-1f8q7fu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/91956/original/image-20150815-5098-1f8q7fu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/91956/original/image-20150815-5098-1f8q7fu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/91956/original/image-20150815-5098-1f8q7fu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/91956/original/image-20150815-5098-1f8q7fu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/91956/original/image-20150815-5098-1f8q7fu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/91956/original/image-20150815-5098-1f8q7fu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/91956/original/image-20150815-5098-1f8q7fu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">This tiny red-billed emerald hummingbird (<em>Chlorostilbon gibsoni</em>) feeds on thousands of flowers a day.</span>
<span class="attribution"><span class="source">Kristiina Hurme</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Tube feeding?</h2>
<p>Hummingbirds’ skinny tongues are about the same length as their bills. They’re perfectly adapted for reaching deep into a flower. <a href="https://books.google.com.co/books/about/The_Naturalists_Library.html?id=tBVmOAAACAAJ&redir_esc=y">For over 180 years</a>, scientists believed that to drink nectar, hummingbirds relied on capillary action. The idea was that their tongues would fill with nectar in the same way a small glass tube fills passively with water. </p>
<p>The physics of capillary action relies on two forces. Adhesion of the liquid molecules to the tube walls makes the liquid climb the sides. Surface tension holds the liquid together and drags the whole fluid column upwards. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/92027/original/image-20150817-5117-1giaoij.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/92027/original/image-20150817-5117-1giaoij.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/92027/original/image-20150817-5117-1giaoij.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=146&fit=crop&dpr=1 600w, https://images.theconversation.com/files/92027/original/image-20150817-5117-1giaoij.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=146&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/92027/original/image-20150817-5117-1giaoij.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=146&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/92027/original/image-20150817-5117-1giaoij.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=184&fit=crop&dpr=1 754w, https://images.theconversation.com/files/92027/original/image-20150817-5117-1giaoij.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=184&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/92027/original/image-20150817-5117-1giaoij.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=184&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A hummingbird’s long skinny tongue has two grooves running down the middle, and ends in a forked tip that spreads inside the nectar.</span>
<span class="attribution"><span class="source">Alejandro Rico-Guevara</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The capillary action theory made sense since a hummingbird’s tongue has two tube-like grooves. It would be a simple, passive way for nectar to travel up the tongue.</p>
<h2>Hummingbirds are faster than that</h2>
<p>But from watching hummingbirds in my (<a href="http://www.alejorico.com/Home.html">Rico-Guevara’s</a>) native Colombia, we felt that capillarity just wasn’t fast enough to keep up with how hummingbirds feed. We predicted that <a href="http://doi.org/10.1073/pnas.1119750109">capillarity was too slow</a> to account for the fast licking rates observed in free-living hummingbirds. Remember, they can drain a flower’s nectar with around 15 licks in under a second!</p>
<p>Four years ago, one of us (Rico-Guevara) and colleague <a href="http://rubegalab.uconn.edu/">Margaret Rubega</a> <a href="http://doi.org/10.1073/pnas.1016944108">challenged the conventional beliefs</a> about capillary action for the first time. We showed that the forked tongue tips are not static, but dramatically spread inside the nectar, with fringed edges that open up like tiny hands. When the hummingbird retracts its tongue from the nectar, these fringes close due to the physical forces of surface tension and <a href="http://www.kruss.de/services/education-theory/glossary/laplace-pressure/">Laplace pressure</a>, trapping nectar drops in their grips. Due to this transformation of the tongue shape, the tongue tips don’t remain in the tube-shape necessary for capillary action. </p>
<p>So how does the rest of the tongue fill with nectar? </p>
<p>We set out to study a medley of hummingbird species to see what these birds were really doing at the flowers. We needed a way to measure a tongue’s thickness during the drinking process – straightforward, but not an easy task.</p>
<p>We designed see-through artificial flowers that we filmed with slow-motion cameras. From these videos, we could then track the shape of the tongue throughout the whole licking cycle. The difficult part was convincing wild hummingbirds to drink on command. Over time, we trained them by habituating them to the phony flower feeders and our whole filming setup.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/91970/original/image-20150815-5095-mndiai.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/91970/original/image-20150815-5095-mndiai.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/91970/original/image-20150815-5095-mndiai.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/91970/original/image-20150815-5095-mndiai.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/91970/original/image-20150815-5095-mndiai.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/91970/original/image-20150815-5095-mndiai.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/91970/original/image-20150815-5095-mndiai.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/91970/original/image-20150815-5095-mndiai.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Wild hummingbirds got used to the bright lights and big cameras – ready to be our movie stars.</span>
<span class="attribution"><span class="source">Kristiina Hurme</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Science discovery via slow motion video</h2>
<p>When a hummingbird inserts its bill into a flower, it still needs to stick its long tongue deeper inside to get at the nectar within. After the tongue fills with nectar, the bird retracts the tongue back inside the bill. <a href="http://www.jstor.org/stable/4085938">Researchers already knew</a> that to keep the nectar inside the beak, the hummingbird squeezes the tongue with the bill tips as it is extended for the next lick. That compresses and flattens the tongue on its way out, leaving the nectar inside the bill. The way in which the nectar is moved from the bill tip to where it can be swallowed <a href="http://digitalcommons.uconn.edu/dissertations/490/">remains unknown</a>. </p>
<p>To study the tongue-filling mechanism, we focused on the flattened shape of the tongue that each lick starts with. If the hummingbirds were using capillarity, once the nectar had made it into the bird’s mouth, the tongue would immediately need to recover its tube-like shape before touching the nectar again. </p>
<p>By closely studying our slow motion videos of the birds drinking at the transparent flowers, we saw that the tongue remained flattened after the squeezing even as it traveled through the air to reach the nectar for another sip. It didn’t snap back to its original pre-drink tube-like shape. </p>
<p>We studied 18 hummingbird species, and in hundreds of licks, we found that the tongue remained flattened until it touches the nectar. This was a key finding because it showed that the tongue didn’t have the empty space inside needed for capillary action to work. Finally, we can confidently rule out capillarity as important for hummingbird drinking.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/QYoYQAbPXbU?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<h2>How they really pump the nectar in</h2>
<p>What we found goes beyond simply debunking capillarity. Hummingbirds have hit on an unexpected way to move liquid very quickly at this micro-scale: their <a href="http://rspb.royalsocietypublishing.org/content/282/1813.cover-expansion">tongues are elastic micropumps</a>.</p>
<p>The grooves in the hummingbird tongue don’t reach the throat, so the bird cannot use them as <a href="http://www.nytimes.com/2009/11/24/science/24obhummingbird.html">tiny straws</a>. For this reason, instead of using vacuum to generate suction – imagine drinking lemonade out of a straw – the system works like a tiny pump, powered by the springiness of the tongue. The bird squashes the tongue flat, and when it springs open, this expansion rapidly pulls the nectar into the grooves in its tongue. It turns out it’s elastic energy – potential mechanical energy stored by the flattening of the tongue – that lets hummingbirds collect nectar much faster than if they relied on capillarity.</p>
<p>While the tongue moves through the air, the elastic energy loaded into the groove walls during the flattening is conserved by a remaining layer of liquid inside the grooves acting as an adhesive. When the tongue touches the nectar, the supply of fluid allows the release of the elastic energy which expands the grooves and pulls the nectar to fill the tongue. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/92159/original/image-20150817-28357-15ep2dx.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/92159/original/image-20150817-28357-15ep2dx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/92159/original/image-20150817-28357-15ep2dx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=459&fit=crop&dpr=1 600w, https://images.theconversation.com/files/92159/original/image-20150817-28357-15ep2dx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=459&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/92159/original/image-20150817-28357-15ep2dx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=459&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/92159/original/image-20150817-28357-15ep2dx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=577&fit=crop&dpr=1 754w, https://images.theconversation.com/files/92159/original/image-20150817-28357-15ep2dx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=577&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/92159/original/image-20150817-28357-15ep2dx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=577&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">As a hummingbird drinks, each lick collects nectar, while rapidly preparing the tongue pump for the next lick.</span>
<span class="attribution"><span class="source">Alejandro Rico-Guevara</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>As biologists, we were excited by this new discovery, but needed the help of an expert in fluid dynamics, <a href="http://www.engr.uconn.edu/%7Ethfan/">Tai-Hsi Fan</a>, to accurately explain the physics of this hummingbird micro-pump, and to make new predictions.</p>
<p>Our research shows how hummingbirds really drink, and provides the first mathematical tools to accurately model their energy intake. These discoveries will influence our understanding of their foraging decisions, ecology and coevolution with the plants they pollinate.</p>
<p>Our ongoing research compares our new model with <a href="http://grantome.com/grant/NSF/IOS-1311443">how much nectar hummingbirds drink at wildflowers</a>, and looks at the trade-offs between <a href="https://www.sciencenews.org/article/hummingbirds-take-stab-rivals-dagger-tipped-bills">drinking efficiently and fighting</a> for dominance over territories either to attract females, to feed, or both.</p><img src="https://counter.theconversation.com/content/46167/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Alejandro Rico Guevara receives funding from the National Science Foundation.</span></em></p><p class="fine-print"><em><span>Kristiina Hurme does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>By filming these ultra-speedy feeders at transparent fake flowers, researchers finally figured out how hummingbirds slurp up the nectar so fast.Alejandro Rico-Guevara, Research Associate in Ecology and Evolutionary Biology, University of ConnecticutKristiina Hurme, Research Associate in Ecology and Evolutionary Biology, University of ConnecticutLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/452952015-08-10T18:01:23Z2015-08-10T18:01:23ZScientists at work: cracking sea lions’ high-thrust, low-wake swimming technique<figure><img src="https://images.theconversation.com/files/91296/original/image-20150810-11088-c74cup.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Observing the foreflipper clap.</span> <span class="attribution"><span class="source">Megan Leftwich</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The California sea lion has a unique way of moving through the ocean. This highly maneuverable aquatic mammal produces thrust primarily with its foreflippers – the ones it has where you have hands. Despite being fast, efficient and agile, this sea lion swimming technique is quite different from the way other large fish and marine mammals move through the water.</p>
<p>It wouldn’t be easy to design a system from scratch that could match the sea lion’s specifications – they produce high levels of thrust while leaving little traceable wake structure. So it makes sense to learn as much as we can about how they do it – with the thought that someday we might be able to engineer something that mimics our biological model. </p>
<p>To understand sea lion hydrodynamics – that is, the physics of how their swimming motion disrupts the surrounding water – we have to first characterize the kinematics – how their bodies move. And to do that we need to observe lots of California sea lion movements. So we head to the zoo.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/91200/original/image-20150807-27600-is2z2t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/91200/original/image-20150807-27600-is2z2t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/91200/original/image-20150807-27600-is2z2t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/91200/original/image-20150807-27600-is2z2t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/91200/original/image-20150807-27600-is2z2t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/91200/original/image-20150807-27600-is2z2t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/91200/original/image-20150807-27600-is2z2t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/91200/original/image-20150807-27600-is2z2t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Visiting the sea lions at the National Zoo with GW undergrad researchers. (Author upper right.)</span>
<span class="attribution"><span class="source">William Atkins/The George Washington University</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>“Field” work close to home</h2>
<p>Typically, fieldwork is hard, time-consuming and expensive. But because our “field” is only two miles away from <a href="https://leftwichlab.seas.gwu.edu/">our lab</a>, and because the <a href="http://nationalzoo.si.edu/Animals/AmericanTrail/">American Trail</a> staff at the <a href="https://nationalzoo.si.edu/">Smithsonian National Zoo</a> is so accommodating, for us it is only hard and time-consuming.</p>
<p>We are able to return time and again to try new techniques and collect more data as needed. To avoid crowds but still get adequate lighting for our cameras, we arrive at 7:30 am to set up and begin taking data.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/91198/original/image-20150807-27571-mq4t2y.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/91198/original/image-20150807-27571-mq4t2y.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/91198/original/image-20150807-27571-mq4t2y.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/91198/original/image-20150807-27571-mq4t2y.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/91198/original/image-20150807-27571-mq4t2y.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/91198/original/image-20150807-27571-mq4t2y.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/91198/original/image-20150807-27571-mq4t2y.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/91198/original/image-20150807-27571-mq4t2y.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Single-camera setup with markers on the glass for filming sea lions.</span>
<span class="attribution"><span class="source">Megan Leftwich</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Our data are high-resolution, high-speed videos. We set up cameras in precise, known locations and place small calibration markers on the viewing window. Multiple cameras are synced using a flash or audio marker – and then we wait.</p>
<p>While this is an “observational” study – we do not mark or touch the animals while obtaining data – the sea lions prefer to play rather than just be observed. So we’ll wave and run back and forth across the viewing window (a great job for undergrad research interns) to entice them to exhibit the behavior we hope to capture. What we really want to see is the sea lion’s propulsive stroke – where they clap their foreflippers toward their belly and glide forward.</p>
<p>So far, in just under two years of collecting data, we have amassed over 100 hours of footage of sea lions swimming, about 30 minutes of which is usable data.</p>
<figure>
<iframe src="https://player.vimeo.com/video/133349233" width="500" height="281" frameborder="0" webkitallowfullscreen="" mozallowfullscreen="" allowfullscreen=""></iframe>
<figcaption><span class="caption">The author describing her research.</span></figcaption>
</figure>
<h2>Back in the lab</h2>
<p>The vast majority of our time is spent not at the zoo with the animals, but with our videos of their movements.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/91201/original/image-20150807-27617-1rt8bsw.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/91201/original/image-20150807-27617-1rt8bsw.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/91201/original/image-20150807-27617-1rt8bsw.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=809&fit=crop&dpr=1 600w, https://images.theconversation.com/files/91201/original/image-20150807-27617-1rt8bsw.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=809&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/91201/original/image-20150807-27617-1rt8bsw.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=809&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/91201/original/image-20150807-27617-1rt8bsw.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1017&fit=crop&dpr=1 754w, https://images.theconversation.com/files/91201/original/image-20150807-27617-1rt8bsw.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1017&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/91201/original/image-20150807-27617-1rt8bsw.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1017&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Ten tracked points on the sea lion’s foreflipper.</span>
<span class="attribution"><span class="source">Megan Leftwich</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Learning something from the data we collect takes time, patience and computers. Of course video is only a two-dimensional representation of what really happened in space. So we convert all our video through a process called digital linear transformation, a method used to <a href="http://dx.doi.org/10.1088/1748-3182/3/3/034001">track three-dimensional motion</a> that was developed by Ty Hedrick of UNC to track hummingbird and hawk moth flying.</p>
<p>Individual points on a sea lion’s flipper are digitally located in each frame of the video (120 frames per second). Those locations are tracked from frame to frame, creating a surface that represents the motion of the sea lion’s foreflipper while swimming.</p>
<p>Through this process, we can create a digital foreflipper that can be programmed to move like a real swimming California sea lion. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/90940/original/image-20150805-22485-puip2u.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/90940/original/image-20150805-22485-puip2u.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/90940/original/image-20150805-22485-puip2u.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/90940/original/image-20150805-22485-puip2u.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/90940/original/image-20150805-22485-puip2u.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/90940/original/image-20150805-22485-puip2u.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=471&fit=crop&dpr=1 754w, https://images.theconversation.com/files/90940/original/image-20150805-22485-puip2u.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=471&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/90940/original/image-20150805-22485-puip2u.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=471&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">We write and use computer codes to track the surface of the foreflipper as it moves.</span>
<span class="attribution"><span class="source">Megan Leftwich</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>So how do they actually move?</h2>
<p>The California sea lion relies predominantly on its foreflippers for thrust production. Thrust is the force that accelerates the animal in the forward direction. The large flippers move through the water in a clapping motion that ends with each flipper pressed against the animal’s torso.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/91301/original/image-20150810-11104-he934g.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/91301/original/image-20150810-11104-he934g.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/91301/original/image-20150810-11104-he934g.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/91301/original/image-20150810-11104-he934g.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/91301/original/image-20150810-11104-he934g.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/91301/original/image-20150810-11104-he934g.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/91301/original/image-20150810-11104-he934g.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/91301/original/image-20150810-11104-he934g.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Watch me clap my foreflippers.</span>
<span class="attribution"><span class="source">Megan Leftwich</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>This flipper-based motion differs significantly from other large fish and marine mammals, which typically have a dominant oscillation frequency. For fish, that means they flap their tails side to side continually. Aquatic mammals flap up and down. In both, every flap takes about the same amount of time. Instead, in sea lions, each clap of the flipper is followed by a prolonged glide — particularly unusual for large, high-thrust-producing swimmers. The smooth swim is assisted by the animal’s low drag coefficient, meaning it glides through the water easily without much resistance slowing it down.</p>
<p>Our observational work so far has led to a <a href="http://dx.doi.org/10.1088/1748-3182/9/4/046010">detailed two-dimensional description</a> of sea lion swimming, and we are currently working to track the flipper in three dimensions.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/91202/original/image-20150807-27612-elucf0.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/91202/original/image-20150807-27612-elucf0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/91202/original/image-20150807-27612-elucf0.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=346&fit=crop&dpr=1 600w, https://images.theconversation.com/files/91202/original/image-20150807-27612-elucf0.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=346&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/91202/original/image-20150807-27612-elucf0.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=346&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/91202/original/image-20150807-27612-elucf0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=435&fit=crop&dpr=1 754w, https://images.theconversation.com/files/91202/original/image-20150807-27612-elucf0.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=435&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/91202/original/image-20150807-27612-elucf0.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=435&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">3D geometry for sea lion foreflipper based on laser scanning.</span>
<span class="attribution"><span class="source">Megan Leftwich</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Creating a robo-foreflipper</h2>
<p>My background, and the focus of my lab, is fluid dynamics, but so far our sea lion studies have been kinematic studies. Ultimately, we want to know how the water around the sea lion reacts to the what we’re learning about how their bodies move. To do that, we are using all the data we’ve collected from the field studies to create a robotic sea lion foreflipper.</p>
<p>The flipper geometry is based on <a href="http://doi.org/10.5226/jabmech.4.25">high-resolution laser scans</a> of a real California sea lion foreflipper. We program its motion based on the results of our field studies. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/91300/original/image-20150810-11107-gi35g4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/91300/original/image-20150810-11107-gi35g4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/91300/original/image-20150810-11107-gi35g4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/91300/original/image-20150810-11107-gi35g4.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/91300/original/image-20150810-11107-gi35g4.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/91300/original/image-20150810-11107-gi35g4.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/91300/original/image-20150810-11107-gi35g4.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/91300/original/image-20150810-11107-gi35g4.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The robotic flipper will be used to measure the reaction of the water to the sea lion’s clapping motion.</span>
<span class="attribution"><span class="source">Megan Leftwich</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>By creating a robotic platform, we have a controllable, scalable device that can be used in the lab. We want to measure how the water reacts to the sea lion flipper, something that’s very hard to figure out using live sea lions, mostly due to their size and the need for highly specialized equipment. </p>
<p>Now with our robo-foreflipper, we can investigate, and hopefully understand, the unique way that sea lions move the water while performing their one-of-a-kind swimming motion. Eventually we might see this technique incorporated into an engineered underwater vehicle that could be used to search for underwater mines, or shipwrecks, or unexplored caves – anything that requires stealth, agility and speed in the water.</p><img src="https://counter.theconversation.com/content/45295/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Megan Leftwich does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The way sea lions swim is unique among fish and marine mammals. Their technique provides a biomechanical model to design agile underwater vehicles… but first we have to figure out how they do it.Megan Leftwich, Assistant Professor of Mechanical and Aerospace Engineering, George Washington UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/337342014-11-09T19:30:11Z2014-11-09T19:30:11ZExplainer: the pitch drop experiment<figure><img src="https://images.theconversation.com/files/63799/original/5vtxb2nn-1415233041.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Dr Andrew Stephenson and Dr Anthony Jacko examine the longest running laboratory experiment in the world.</span> <span class="attribution"><span class="license">Author provided</span></span></figcaption></figure><p>Something strange is happening within the world-famous <a href="http://smp.uq.edu.au/content/pitch-drop-experiment">pitch drop experiment</a> with the latest drop forming much faster than the last couple of drops.</p>
<p>There have been nine drops so far and all attention is now on trying to observe the <a href="http://www.thetenthwatch.com/">tenth</a>, expected sometime in the 2020s.</p>
<p>The actual experiment began in October 1930 and is now recognised by Guinness World Records as the <a href="http://www.guinnessworldrecords.com.au/world-records/2000/longest-running-laboratory-experiment">longest-running laboratory experiment</a> – and in all that time no one has ever witnessed a single drop of pitch to fall.</p>
<p>But what started as a simple lecture demonstration has captured the interest of tens of thousands of people worldwide.</p>
<h2>In the beginning</h2>
<p>The experiment’s creator and its first custodian was Professor Thomas Parnell, the first professor of physics at The University of Queensland. In 1927 he setup the experiment to demonstrate the viscosity of pitch – the thickest fluid known to exist.</p>
<p>Professor Parnell heated some pitch and poured it into a glass funnel, then left it to cool for three years. In October 1930 the bottom of the funnel was cut and the pitch began to flow.</p>
<p>It took eight years for the first drop to fall. The next five drops each took seven to nine years to fall, with the sixth drop falling in April 1979.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/63707/original/pxwphrn2-1415163538.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/63707/original/pxwphrn2-1415163538.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/63707/original/pxwphrn2-1415163538.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=375&fit=crop&dpr=1 600w, https://images.theconversation.com/files/63707/original/pxwphrn2-1415163538.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=375&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/63707/original/pxwphrn2-1415163538.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=375&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/63707/original/pxwphrn2-1415163538.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=472&fit=crop&dpr=1 754w, https://images.theconversation.com/files/63707/original/pxwphrn2-1415163538.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=472&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/63707/original/pxwphrn2-1415163538.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=472&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">After the sixth pitch drop fell in 1979.</span>
<span class="attribution"><span class="source">University of Queensland</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The seventh drop fell in July 1988 – nine years after the sixth drop – and it look liked the pitch drop experiment had revealed all it had to offer.</p>
<p>But rarely does science go to plan. In what could be described as relatively recent times the pitch drop experiment has become far less predictable.</p>
<h2>The troublesome eighth drop</h2>
<p>The eighth drop took more than 12 years to fall. No one was quite sure why it took so long. Perhaps it was due to decreasing pressure from the ever-diminishing mass of pitch remaining in the funnel.</p>
<p>Another explanation was that the building housing the pitch drop experiment had been renovated in the 1980s and it was thought that the newly installed air-conditioning was cooling the experiment down, thus making the pitch more viscous.</p>
<p>Evidence supporting this hypothesis was the physical size of the eighth drop, which was noticeably larger than previous drops.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/63691/original/3pfgktbb-1415161031.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/63691/original/3pfgktbb-1415161031.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/63691/original/3pfgktbb-1415161031.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=612&fit=crop&dpr=1 600w, https://images.theconversation.com/files/63691/original/3pfgktbb-1415161031.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=612&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/63691/original/3pfgktbb-1415161031.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=612&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/63691/original/3pfgktbb-1415161031.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=769&fit=crop&dpr=1 754w, https://images.theconversation.com/files/63691/original/3pfgktbb-1415161031.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=769&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/63691/original/3pfgktbb-1415161031.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=769&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Professor John Mainstone with the eighth drop forming in 1990.</span>
<span class="attribution"><span class="source">University of Queensland</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>It was hoped that the ninth drop might shed some light on this puzzle, but as the ninth drop began to descend it ran into the tail of the eighth drop sitting in the beaker below.</p>
<p>Rather than altering the experiment, Professor John Mainstone – the pitch drop’s second custodian – thought it best to leave the pitch drop as Professor Parnell had created it to see what would happen.</p>
<p>After a decade it was apparent that the ninth drop was bigger and falling slower than the first seven drops. But this didn’t help solve unanswered questions as the experimental conditions had changed – the ninth drop was being supported by the eighth. </p>
<p>Such is the nature of science that more spanners were found to have been thrown in the works. At the start of 2012 I arranged for a video camera to record the pitch drop experiment and make a time-lapse video so that it would be possible to actually see the pitch falling.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/UZKZF7FNh_0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>By mid-2013 it became apparent that the ninth drop wasn’t going to actually drop – rather it was going to make a gentle touchdown. </p>
<p>Although it was disappointing knowing that the ninth drop wasn’t going to fall, the pitch drop was still providing new insights. The stem of the eighth drop was buckling and breaking, the time-lapse was revealing that the ninth drop’s descent was accelerating and there was still the goal of being the first person in history to witness a drop of pitch come to its end.</p>
<h2>Still no witnesses</h2>
<p>Since the pitch drop experiment began in 1930 no one has ever seen the pitch actually drop. Professors Parnell and Mainstone knew the date of drops simply because the drop was attached one day, and had fallen the next.</p>
<p>A computer and camera were set up to capture the eighth drop in 2000, but an unfortunately-timed blackout prevented the drop from being recorded.</p>
<p>Although the ninth drop wasn’t actually going to fall there was a lot of interest from around the globe (31,335 people from 158 countries registered to view a live video stream) as to the exact moment the ninth drop would touch down.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/63704/original/kg6tz279-1415162666.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/63704/original/kg6tz279-1415162666.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/63704/original/kg6tz279-1415162666.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=772&fit=crop&dpr=1 600w, https://images.theconversation.com/files/63704/original/kg6tz279-1415162666.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=772&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/63704/original/kg6tz279-1415162666.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=772&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/63704/original/kg6tz279-1415162666.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=970&fit=crop&dpr=1 754w, https://images.theconversation.com/files/63704/original/kg6tz279-1415162666.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=970&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/63704/original/kg6tz279-1415162666.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=970&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Waiting for the next pitch drop to fall.</span>
<span class="attribution"><span class="source">University of Queensland</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>With the time-lapse camera I had set up, which was also recording continuous video of the four most recent days, and two additional video cameras that were set up in anticipation of the ninth drop’s demise it was thought the exact moment of touch down would be known for the first time in history.</p>
<p>But the pitch drop experiment had other plans.</p>
<p>When something, such as water or pitch, drips the drop starts with zero velocity, then freefalls accelerating at 9.8m/s<sup>2</sup>, and comes to a stop when it crashes to the ground.</p>
<p>Determining when this happens is easy because of the noticeable changes in velocity. Unfortunately, the ninth drop wasn’t going to freefall, so it wouldn’t have a sudden increase in speed.</p>
<p>Making matters worse was the fact that the ninth drop wasn’t landing squarely on top of the eighth drop – it was going to make a glancing blow and continue to descend after touching down.</p>
<p>This meant that determining the moment the ninth drop touches down was a matter of noticing its rate of descent change from an incredibly slow velocity to an even slower one.</p>
<p>No one on earth could possibly hope to see when that change occurred. The best guess of observers initially had the time of touchdown to within a four-day period of April 11-14 2014.</p>
<p>It was hoped that touchdown could be seen in the time-lapse video – which was constructed from a series of still images taken once per day thereby speeding up the movement two million times.</p>
<p>Unfortunately the eighth drop was slightly closer to the camera than the ninth drop, making their point of contact obscured from view. But with analysis of the time-lapse it is possible to determine the date of touchdown.</p>
<p>In an analysis by Dr Anthony Jacko and myself the plot (below) shows the position of the drop April 4-23. The slope of the data indicates the speed at which the pitch is descending.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/63524/original/kfmhjrxg-1414993814.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/63524/original/kfmhjrxg-1414993814.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/63524/original/kfmhjrxg-1414993814.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=403&fit=crop&dpr=1 600w, https://images.theconversation.com/files/63524/original/kfmhjrxg-1414993814.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=403&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/63524/original/kfmhjrxg-1414993814.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=403&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/63524/original/kfmhjrxg-1414993814.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=506&fit=crop&dpr=1 754w, https://images.theconversation.com/files/63524/original/kfmhjrxg-1414993814.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=506&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/63524/original/kfmhjrxg-1414993814.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=506&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">When was the ninth drop to fall in the Pitch Drop Experiment?</span>
<span class="attribution"><span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>A discontinuity can be seen around the April 11/12 where the pitch changes from descending at a rate of 0.25mm a day (blue line) to 0.1mm a day (green line), indicating that touchdown occurred on April 11 or 12.</p>
<p>The exact moment can be further constrained as I was fortunate enough to closely examine the pitch drop at around midday of the 11th of April (after the photo from the 11th was taken) and a small gap – just a fraction of a millimetre – was still visible between the ninth and eighth drops.</p>
<p>Given the rate of descent indicated in the figure above it is concluded that touchdown occurred on April 12.</p>
<p>Although this clears up the issue of when the ninth drop touched down, more questions about the Pitch Drop Experiment have arisen.</p>
<p>After the eighth drop touched down without breaking, the second and third custodians – Professors Mainstone and White – had many conversations about what to do if the ninth drop also touched.</p>
<p>One thing they considered early on was lifting the glass funnel to provide more space for the drops to form – and then hopefully break away. But by the time they isolated this as a course of action it was too late as the eighth drop appeared to be cemented to the drop pile beneath it.</p>
<h2>The pitch drop uncovered</h2>
<p>Not wishing a recurrence with the ninth drop, on April 24 this year Professor White decided to replace the beaker – he took off the bell jar covering the pitch drop experiment for the first time in decades.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/63701/original/mfkw8xbm-1415162416.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/63701/original/mfkw8xbm-1415162416.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/63701/original/mfkw8xbm-1415162416.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=384&fit=crop&dpr=1 600w, https://images.theconversation.com/files/63701/original/mfkw8xbm-1415162416.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=384&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/63701/original/mfkw8xbm-1415162416.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=384&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/63701/original/mfkw8xbm-1415162416.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=482&fit=crop&dpr=1 754w, https://images.theconversation.com/files/63701/original/mfkw8xbm-1415162416.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=482&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/63701/original/mfkw8xbm-1415162416.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=482&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Professor White, the third and current custodian of the pitch drop experiment.</span>
<span class="attribution"><span class="source">University of Queensland</span>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Unbeknown to him, or anyone alive, was that there was a seal between the glass bell jar and the wooden platform upon which the experiment sits, and that seal had degraded over time.</p>
<p>Instead of the bell jar lifting off as expected, the entire experimental setup briefly lifted up with the jar before separating and falling back down. This caused the ninth drop’s stem to separate completely from the funnel from which it was falling.</p>
<p>When viewing the broken stem something rather curious was observed. There was a depression in the centre of the stem indicating that the ninth drop wasn’t being supported by the entire circular cross-section of the cylindrical stem from above, but rather an outer ring as the stem at the point of breaking was hollow.</p>
<h2>Things are speeding up</h2>
<p>Another curious observation is that the tenth drop can be observed to be forming quicker than expected. Analysing the first six months of time-lapse footage reveals that the tenth drop’s volume is increasing at a rate of approximately 19 mm<sup>3</sup> a day – that’s about one cubic centimetre every 50 days.</p>
<p>In 1984 the first, and only, <a href="http://iopscience.iop.org/0143-0807/5/4/003/">paper published</a> about the pitch drop experiment stated that the viscosity of the pitch (averaged over the first six drops) was 2.3×10<sup>8</sup> Pa s (pascal seconds) – that’s about 250 billion times that of water.</p>
<p>Using the same dimensions as those in the paper I have calculated that the viscosity of the tenth drop is 2.7×10<sup>7</sup> Pa s – about 30 billion times that of water.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/63800/original/n6srhm3k-1415233572.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/63800/original/n6srhm3k-1415233572.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/63800/original/n6srhm3k-1415233572.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=343&fit=crop&dpr=1 600w, https://images.theconversation.com/files/63800/original/n6srhm3k-1415233572.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=343&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/63800/original/n6srhm3k-1415233572.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=343&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/63800/original/n6srhm3k-1415233572.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=431&fit=crop&dpr=1 754w, https://images.theconversation.com/files/63800/original/n6srhm3k-1415233572.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=431&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/63800/original/n6srhm3k-1415233572.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=431&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Waiting for the tenth drop to fall.</span>
<span class="attribution"><span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Although this new value is an order of magnitude smaller than the old value – and would explain the apparent faster flow rate – it is within the range of values predicted by the original paper, using a model that included daily and seasonal temperature variations.</p>
<p>Another recent development for the pitch drop experiment, which could explain the increased flow rate of the tenth drop, is the lighting in the experiment’s display cabinet.</p>
<p>In April 2010 I arranged for fluorescent down-lights to be installed in the display cabinet. These were changed some years later to much hotter halogens.</p>
<p>As the pitch drop experiment has a fan base from all over the world who view it live over the internet it was necessary for the lights to be on continuously. But this increased the temperature in the display case and to make matters worse the pitch drop is sealed inside its very own greenhouse – the bell jar. </p>
<p>On October 17 this year, Professor White and I took several temperature measurements which revealed that the pitch drop experiment was above room temperature, by several degrees, and varied, with the pitch at the top of the funnel being almost ten degrees hotter than the pitch in the drop.</p>
<p>The increased temperature will have increased the flow rate, which could explain the tenth drop’s lower viscosity and speedy formation.</p>
<p>How much effect the raised temperature has had is hard to say, since temperature records have never been kept as the pitch drop experiment was started simply as a demonstration, but it is something we will find out.</p>
<p>The down-lights have now been changed to much cooler LED lights, which have brought the pitch drop experiment back down to room temperature.</p>
<p>Hopefully in another six months there will be enough data to make a comparison, but then again maybe the pitch drop may have other plans and it may take longer.</p>
<p>After all, there is no telling how many more custodians the pitch drop experiment will go through before it has finished revealing all its secrets. I guess we will just have to wait and see.</p>
<p><strong><em>Keep an eye on the pitch drop experiment on the <a href="http://www.thetenthwatch.com/">Tenth Watch website</a>.</em></strong></p><img src="https://counter.theconversation.com/content/33734/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Stephenson does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Something strange is happening within the world-famous pitch drop experiment with the latest drop forming much faster than the last couple of drops. There have been nine drops so far and all attention…Andrew Stephenson, Science Communicator, The University of QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/195752013-10-29T14:56:57Z2013-10-29T14:56:57ZIt’s no reverse microwave, but it is cool<figure><img src="https://images.theconversation.com/files/34028/original/q6422cwn-1383055798.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Better than your refrigerator</span> <span class="attribution"><span class="source">Eco-Cool</span></span></figcaption></figure><p>How can you turn lukewarm lager to ice-cold beer in under a minute? A startup has developed a nifty gizmo which does just that, saving both energy and embarrassment at parties. Manufacturer Enviro-Cool <a href="http://www.enviro-cool.co.uk/technical-information/">claims</a> that chilling on demand with a V-Tex could save retailers €1000 per fridge per year, and of course help to keep the planet cool too. So how does the device actually work?</p>
<p>Media reports have dubbed the device a “<a href="http://www.telegraph.co.uk/science/science-news/10404692/Reverse-microwave-can-chill-wine-bottles-and-fizzy-drink-cans-in-45-seconds.html">reverse microwave</a>”, but that analogy would receive a chilly reception amongst physicists. Unfortunately, you can’t simply wire up a microwave oven backwards and suck the heat from an object. </p>
<p>In fact, despite the PR spin about “Rankine vortices”, this device is remarkably unremarkable in some respects: the rapid cooling of drinks is achieved by putting them into contact with something cold. However, there is a twist: the interesting science here is fluid dynamics, not thermodynamics.</p>
<h2>Tricky chilling</h2>
<p>It’s easy to heat food quickly in a microwave oven. Why is it so hard to cool things down? The temperature of an object is essentially a measure of how much energy it holds. A hotter object has more energy than a colder one. Cooling is difficult because coaxing the atoms inside an object to give up their energy is a tricky business. </p>
<p>If you want to cool a material at will, you need to choose your material quite carefully. A gas is ideal: gases can be heated by compression (which is why a bicycle pump is warm to the touch after use) or, conversely, cooled by expansion (which is why the rapidly-expanding gas from an aerosol can feels cool).</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/34024/original/vntkwsh8-1383049679.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/34024/original/vntkwsh8-1383049679.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/34024/original/vntkwsh8-1383049679.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/34024/original/vntkwsh8-1383049679.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/34024/original/vntkwsh8-1383049679.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=565&fit=crop&dpr=1 754w, https://images.theconversation.com/files/34024/original/vntkwsh8-1383049679.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=565&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/34024/original/vntkwsh8-1383049679.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=565&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Chilled in a few seconds.</span>
<span class="attribution"><span class="source">Eco-Cool</span></span>
</figcaption>
</figure>
<p>There are no gaseous foods, and solids or liquids are more difficult to chill. The only simple option is to place them in contact with something cold.</p>
<p>This is, of course, how a fridge works: compressing and expanding gas in a series of tubes makes them cold. These tubes then cool the air in the fridge, which then cools your food. The problem with this process is that air and food are pretty terrible conductors of heat, so it takes a long time for the heat to flow out of food, via the air and into the cold pipes, where it is expelled from the back of the fridge to warm up the kitchen.</p>
<p>Thus, to increase the speed of cooling, we need another medium to transport heat. This is the first aspect of the V-Tex which differs from a normal fridge: it uses water to carry the heat from the drink being cooled. But water is so much more effective than air that you run into another problem. Suck out heat from a chicken too quickly and the skin will be frozen before the inside even begins to cool—the opposite of a typical barbecue disaster where food cooked at too high a heat is burnt on the outside, but still raw on the inside. In the case of drinks, the nonuniform cooling can create either an exterior layer of ice with a highly concentrated solution of icky syrup at its core, or an unintentional slushie of half-frozen Sauvignon blanc.</p>
<p>With solid objects, from last night’s pasta bake to <a href="http://www.thenakedscientists.com/HTML/content/interviews/interview/1802/">organs for transplant</a>, this is where the story ends: you are just going to have to cool it more carefully if you want to avoid freezing. But with a liquid, you have another option: agitate the liquid such that the whole volume is uniformly exposed to the cold.</p>
<h2>Twisted problem</h2>
<p>Many common beverages, however, pose one further problem. From Pepsi to Prosecco, the fizz in fizzy drinks comes from CO<sub>2</sub> gas dissolved in the liquid. This CO<sub>2</sub> is looking for any excuse to escape, and these excuses come in the form of “nucleation sites”, which encourage bubbles to form: from <a href="http://www.youtube.com/watch?v=hKoB0MHVBvM">tiny pits on the surface of Mentos</a> to a disturbance in the liquid itself. This is why you can’t simply shake ’n’ cool: if you’ve ever played a playground prank with a shaken bottle of Coke, or watched champagne being sprayed from a Formula One podium, you’ll be aware of the effervescent consequences of disturbing a liquid containing dissolved gas.</p>
<p>This is why the V-Tex designers had to devise a smart way of uniformly cooling fizzy liquids. The solution was to rotate, shake with a wiggle and rotate again. This creates a smooth-flowing vortex, with no pressure waves which might induce bubble formation. Details are scant (patents cover the meticulous choreography behind it), but the website does <a href="http://www.enviro-cool.co.uk/ip/">mention</a> repeated creation and destruction of a “Rankine vortex”, which is one way in which a fluid can smoothly swirl.</p>
<p>This device is no reverse microwave: its rapid cooling only works on liquids, and the thermal conduction of the container makes a significant difference (<a href="http://www.enviro-cool.co.uk/cooling-times/">they claim</a> a 500 ml metal can can be cooled in 50 seconds and an equivalent glass bottle would take six minutes).</p>
<p>The media missed out on a better story: in a V-Tex your drink is being stirred, not shaken, by a rapidly moving robot arm (in a tank of ice water). Make the whole assembly transparent and throw in some LEDs, a little more like <a href="http://www.enviro-cool.co.uk/prototype-gallery/">the prototype</a>, and the short wait for your fizzy pop lays bare some cool physics.</p><img src="https://counter.theconversation.com/content/19575/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Steele does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>How can you turn lukewarm lager to ice-cold beer in under a minute? A startup has developed a nifty gizmo which does just that, saving both energy and embarrassment at parties. Manufacturer Enviro-Cool…Andrew Steele, Bioinformatician, King's College LondonLicensed as Creative Commons – attribution, no derivatives.