tag:theconversation.com,2011:/au/topics/phase-transition-14577/articlesPhase transition – The Conversation2021-03-18T19:01:48Ztag:theconversation.com,2011:article/1445662021-03-18T19:01:48Z2021-03-18T19:01:48ZCurious Kids: how do freezers work?<figure><img src="https://images.theconversation.com/files/388723/original/file-20210310-23-xt7gt6.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C3872%2C2585&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><strong>How does the freezer work? — Leon, aged 4</strong></p>
<p><a href="https://theconversation.com/au/topics/curious-kids-36782"><img src="https://images.theconversation.com/files/291898/original/file-20190911-190031-enlxbk.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=90&fit=crop&dpr=1" width="100%"></a></p>
<p>Hi Leon,</p>
<p>That’s a great question! But freezers are a bit tricky to explain, so we’ll need to talk about a few other things first.</p>
<p>Everything you can touch and feel (like air, water, rocks and mice) is made of tiny balls called <em>atoms</em>. When atoms join up into small groups moving around together, they are called <em>molecules</em>. Atoms and molecules are too small to see without very powerful microscopes.</p>
<h2>Solids, liquids and gases</h2>
<p>Most things come in three <em>phases</em>: solid, liquid or gas. Think of ice, water and steam. If a gas is not too hot, we can also call it <em>vapour</em>. (There are other phases too, but let’s ignore them for today.)</p>
<p>In solids (like ice), atoms or molecules are tightly stuck together and can barely move. They are usually lined up in neat rows called <em>crystals</em>. In liquids (like water) atoms or molecules are loosely stuck close together, but can move around. In a gas (like steam), atoms or molecules are far apart and free to float away from each other.</p>
<p>Most gases, including air, are made of small molecules. Some gases (like the helium inside floating party balloons) are made of single atoms moving around on their own.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/388238/original/file-20210308-15-1jy831a.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/388238/original/file-20210308-15-1jy831a.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=487&fit=crop&dpr=1 600w, https://images.theconversation.com/files/388238/original/file-20210308-15-1jy831a.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=487&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/388238/original/file-20210308-15-1jy831a.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=487&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/388238/original/file-20210308-15-1jy831a.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=612&fit=crop&dpr=1 754w, https://images.theconversation.com/files/388238/original/file-20210308-15-1jy831a.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=612&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/388238/original/file-20210308-15-1jy831a.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=612&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A solid melts into a liquid then evaporates into a gas (or vapour)</span>
<span class="attribution"><span class="source">Stephen G Bosi</span></span>
</figcaption>
</figure>
<p>If I heat up a solid, the atoms or molecules start to bounce a little bit, but they still stay stuck in their neat rows. Now, if I add an extra burst of heat, the solid turns into liquid. This means the atoms and molecules bounce around so hard they start to move around, breaking up those neat rows. Although the atoms can now flow around, they still stay very close together. This is what’s happening if you put an ice block in a bowl and watch it slowly melt into water.</p>
<p>To turn a liquid into a gas (or vapour), the atoms and molecules must break away completely from their neighbours. This takes another extra burst of heat to give the atoms and molecules a kick to rip them away from their sticky neighbours and float away. (Scientists call this extra burst of heat <em>latent heat</em>.) </p>
<p>This is what happens when you put water into a kettle, turn on the heat, and watch the steam floating out of the spout.</p>
<p>These atoms or molecules carry that extra burst of heat away with them when they float away. This is why your face feels cooler if the wind turns your sweat into vapour and floats away from your face.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-why-can-some-cups-go-in-the-microwave-and-some-not-82831">Curious Kids: Why can some cups go in the microwave and some not?</a>
</strong>
</em>
</p>
<hr>
<p>OK. Now let’s try it <em>backwards</em>. If you take enough heat out of a vapour (like steam), it will turn back into a liquid (like water). Whenever this happens, the vapour brings the extra burst of heat back into the liquid.</p>
<p>Now, finally, I can explain how your freezer works.</p>
<h2>How the freezer works… at last!</h2>
<p>Hidden inside the walls of your freezer is a curly metal tube called a cooling pipe. It is full of a special liquid that evaporates easily. </p>
<p>The cooling pipe is connected to a pump that sucks in vapour from the cooling pipe. The sucking makes more liquid turn to vapour, and when that happens it takes some heat out of the freezer. Just like sweat floating away cools your face down, this vapour floating away makes the inside of the freezer cool down.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/388239/original/file-20210308-13-139tnur.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/388239/original/file-20210308-13-139tnur.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=287&fit=crop&dpr=1 600w, https://images.theconversation.com/files/388239/original/file-20210308-13-139tnur.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=287&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/388239/original/file-20210308-13-139tnur.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=287&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/388239/original/file-20210308-13-139tnur.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=361&fit=crop&dpr=1 754w, https://images.theconversation.com/files/388239/original/file-20210308-13-139tnur.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=361&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/388239/original/file-20210308-13-139tnur.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=361&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The cooling system inside a freezer.</span>
<span class="attribution"><span class="source">Stephen G Bosi</span></span>
</figcaption>
</figure>
<p>Next, the pump takes vapour from the cooling pipe and squeezes it into another curly pipe on the outside of the back of the fridge. When the pump squeezes the vapour, it pushes the molecules closer together so they start to stick together and turn into a liquid again.</p>
<p>When the gas turns back into a liquid, it gives off the latent heat energy it took from the freezer. So the pipe on the back of the fridge gets warm, and the heat escapes into the air in your kitchen.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/curious-kids-how-does-heat-travel-through-space-if-space-is-a-vacuum-111889">Curious Kids: how does heat travel through space if space is a vacuum?</a>
</strong>
</em>
</p>
<hr>
<p>In other words, the pump moves heat from inside your freezer and lets it go into your kitchen, making the freezer colder and your kitchen warmer. If you feel the back and sides of your fridge, they should feel a bit warm. That’s the heat that used to be inside your freezer!</p>
<p>After releasing its heat energy, the liquid leaks through a little skinny pipe back into the cooling pipe where it started. Then the sucking from the pump turns it into gas again, and the whole cycle repeats over and over. And that’s what keeps your freezer cold.</p>
<hr>
<p><em>Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to curiouskids@theconversation.edu.au</em></p><img src="https://counter.theconversation.com/content/144566/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Stephen G Bosi 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>Freezers use a evaporation and condensation to pump heat out of the fridge and into the kitchen.Stephen G Bosi, Senior Lecturer in Physics, University of New EnglandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/703562016-12-22T13:18:57Z2016-12-22T13:18:57ZScientists have discovered a new state of matter for water<figure><img src="https://images.theconversation.com/files/151055/original/image-20161220-26738-gfulge.jpg?ixlib=rb-1.1.0&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>One of the most basic things we are taught in school science classes is that water can exist in three different states, either as solid ice, liquid water, or vapour gas. But an international team of scientists have recently found signs that liquid water might actually come in two different states.</p>
<p>Writing in an experimental paper, published in the <a href="http://www.inderscience.com/offer.php?id=79670">International Journal of Nanotechnology</a>, the researchers were surprised to find a number of physical properties of water change their behaviour between 50°C and 60°C. This sign of a potential change to a second liquid state could spark a heated discussion in the scientific community. And, if confirmed, it could have implications for a range of fields, including nanotechnology and biology.</p>
<p><a href="https://www.grc.nasa.gov/www/k-12/airplane/state.html">States of matter</a>, also called “phases”, are a key concept in the study of systems made from atoms and molecules. Roughly speaking, a system formed from many molecules can be arranged in a certain number of configurations depending on its total energy. At higher temperatures (and therefore higher energies), the molecules have more possible configurations and so are more disorganised and can move about relatively freely (the gas phase). At lower temperatures, the molecules have a more limited number of configurations and so form a more ordered phase (a liquid). If the temperature goes down further, they arrange themselves in a very specific configuration, producing a solid.</p>
<p>This picture is common for relatively simple molecules such as carbon dioxide or methane, which have three clear, different states (liquid, solid and gas). But for more complex molecules, there is a larger number of possible configurations and this gives rise to more phases. A beautiful illustration of this is the rich behaviour of liquid crystals, which are formed by complex organic molecules and can flow like liquids, but still have a <a href="http://www.nobelprize.org/educational/physics/liquid_crystals/history/">solid-like crystalline structure</a></p>
<p>Because the phase of a substance is determined by how its molecules are configured, many physical properties of that substance will change abruptly as it goes from one state to another. In the recent paper, the researchers measured several telltale physical properties of water at temperatures between 0°C and 100°C under normal atmospheric conditions (meaning the water was a liquid). Surprisingly, they found a kink in properties such as the water’s surface tension and its refractive index (a measure of how light travels through it) at around 50°C.</p>
<h2>Special structure</h2>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/151259/original/image-20161221-4090-kxagye.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/151259/original/image-20161221-4090-kxagye.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=363&fit=crop&dpr=1 600w, https://images.theconversation.com/files/151259/original/image-20161221-4090-kxagye.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=363&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/151259/original/image-20161221-4090-kxagye.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=363&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/151259/original/image-20161221-4090-kxagye.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=456&fit=crop&dpr=1 754w, https://images.theconversation.com/files/151259/original/image-20161221-4090-kxagye.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=456&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/151259/original/image-20161221-4090-kxagye.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=456&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Hydrogen bonds.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Miri9.jpg">Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>How can this be? The structure of a water molecule, H₂O, is very interesting and can be pictured like a sort of arrow tip, with the two hydrogen atoms flanking the oxygen atom at the top. The electrons in the molecule tend to be distributed in a rather asymmetric way, making the oxygen side negatively charged relative to the hydrogen side. This simple structural feature leads to a kind of interaction between water molecules known as hydrogen bonding, in which the opposite charges attract each other.</p>
<p>This gives water properties that, in many cases, break the trends observed for other simple liquids. For example, unlike most other substances, a fixed mass of water takes up more room as a solid (ice) than as a (liquid) because of the way it molecules form a specific regular structure. Another example is the surface tension of liquid water, which is roughly twice that of other non-polar, simpler, liquids.</p>
<p>Water is simple enough, but not too simple. This means that one possibility for explaining the apparent extra phase of water is that it behaves a little bit like a liquid crystal. The hydrogen bonds between molecules keep some order at low temperatures, but eventually could take a second, less-ordered liquid phase at higher temperatures. This could explain the kinks observed by the researchers in their data. </p>
<p>If confirmed, the authors’ findings could have many applications. For example, if changes in the environment (such as temperature) cause changes in a substance’s physical properties, then this can potentially be used for sensing applications. Perhaps more fundamentally, biological systems are mostly made of water. How biological molecules (such as proteins) interact with each other likely depends on the specific manner in which water molecules arrange to form a liquid phase. Understanding how water molecules arrange themselves on average at different temperatures could shed light on the workings of how they interact in biological systems.</p>
<p>The discovery is an exciting opportunity for theorists and experimentalists, and a beautiful example of how even the most familiar substance still has secrets hiding within.</p><img src="https://counter.theconversation.com/content/70356/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rodrigo Ledesma-Aguilar 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>Liquid water develops different properties above around 50°C.Rodrigo Ledesma-Aguilar, Senior Lecturer in Physics and Electrical Engineering, Northumbria University, NewcastleLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/665432016-10-08T00:51:40Z2016-10-08T00:51:40ZPhysicists explore exotic states of matter inspired by Nobel-winning research<figure><img src="https://images.theconversation.com/files/140958/original/image-20161007-21414-ajgt9v.jpg?ixlib=rb-1.1.0&rect=61%2C267%2C284%2C181&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Things are kind of different on the quantum level.</span> <span class="attribution"><span class="source">Nandini Trivedi</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>The 2016 Nobel Prize in physics has been awarded to <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2016/thouless-facts.html">David Thouless</a>, <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2016/haldane-facts.html">Duncan Haldane</a> and <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2016/kosterlitz-facts.html">Michael Kosterlitz</a>, three theoretical physicists whose research used the unexpected mathematical lens of <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2016/popular-physicsprize2016.pdf">topology to investigate phases of matter and the transitions between them</a>.</p>
<p>Topology is a branch of mathematics that deals with understanding shapes of objects; it’s interested in “invariants” that don’t change when a shape is deformed, like the number of holes an object has. Physics is the study of matter and its properties. The Nobel Prize winners were the first to make the connection between these two worlds.</p>
<p>Everyone is used to the idea that a material can take various familiar forms such as a solid, liquid or gas. But the Nobel Prize recognizes other surprising phases of matter – called topological phases – that the winners proposed theoretically and experimentalists have since explored.</p>
<p>Topology is opening up new platforms for observing and understanding these new states of matter in many branches of physics. I work with theoretical aspects of cold atomic gases, a field which has only developed in the years since Thouless, Haldane and Kosterlitz did their groundbreaking theoretical work. Using lasers and atoms to emulate complex materials, cold atom researchers have begun to realize some of the laureates’ predictions – with the promise of much more to come.</p>
<h2>Cold atoms get us to quantum states of matter</h2>
<p>All matter is made up of building blocks, such as atoms. When many atoms come together in a material, they start to interact. As the temperature changes, the state of matter starts to change. For instance, water is a liquid until a fixed temperature, when it turns into vapor (373 degrees Kelvin; 212 degrees Fahrenheit; 100 degrees Celsius); and if you cool, solid ice forms at a fixed temperature (273K; 32°F; 0°C). The laws of physics give us a theoretical limit to how low the temperature can get. This lowest possible temperature is called absolute zero (0K) (and equals -460°F or -273°C).</p>
<p>Classical physics governs our everyday world. Classical physics tells us that if we cool atoms to really low temperatures, they stop their normally constant vibrating and come to a standstill.</p>
<p>But really, as we cool atoms down to temperatures approaching close to 0K, we leave the regime of classical physics – quantum mechanics begins to govern what we see. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/nAGPAb4obs8?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Atoms start to behave not as individual particles but as waves in the world of quantum physics.</span></figcaption>
</figure>
<p>In the quantum mechanical world, if an object’s position becomes sharply defined then its momentum becomes highly uncertain, and vice versa. Thus, if we cool atoms down, the momentum of each atom decreases, and the quantum uncertainty of its position grows. Instead of being able to pinpoint where each atom is, we can now only see a blurry space somewhere within which the atom must be. At some point, the neighboring uncertain positions of nearby atoms start overlapping and the atoms lose their individual identities. Surprisingly, the distinct atoms become a single entity, and behave as <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/2001/">one coherent unit</a> – a discovery that won a previous Nobel.</p>
<p>This new, amazing way atoms organize themselves at very low temperatures results in new properties of matter; it’s no longer a classical solid in which the atoms occupy periodic well-defined positions, like eggs in a carton.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/140881/original/image-20161007-21416-dcixy9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/140881/original/image-20161007-21416-dcixy9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/140881/original/image-20161007-21416-dcixy9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=581&fit=crop&dpr=1 600w, https://images.theconversation.com/files/140881/original/image-20161007-21416-dcixy9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=581&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/140881/original/image-20161007-21416-dcixy9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=581&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/140881/original/image-20161007-21416-dcixy9.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=730&fit=crop&dpr=1 754w, https://images.theconversation.com/files/140881/original/image-20161007-21416-dcixy9.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=730&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/140881/original/image-20161007-21416-dcixy9.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=730&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Supercooled atoms are highly coherent.</span>
<span class="attribution"><span class="source">Nandini Trivedi</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Instead, the material is now in a new quantum state of matter in which each atom has become a wave with its position no longer identifiable. And yet the atoms are not moving around chaotically. Instead, they are highly coherent, with a new kind of quantum order. Just like laser beams, the coherent matter waves of superfluids, superconductors and magnets <a href="http://doi.org/10.1126/science.275.5300.637">can produce interference patterns</a>.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/140883/original/image-20161007-21454-154tkox.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/140883/original/image-20161007-21454-154tkox.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/140883/original/image-20161007-21454-154tkox.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=586&fit=crop&dpr=1 600w, https://images.theconversation.com/files/140883/original/image-20161007-21454-154tkox.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=586&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/140883/original/image-20161007-21454-154tkox.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=586&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/140883/original/image-20161007-21454-154tkox.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=737&fit=crop&dpr=1 754w, https://images.theconversation.com/files/140883/original/image-20161007-21454-154tkox.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=737&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/140883/original/image-20161007-21454-154tkox.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=737&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 temperatures rise, materials lose their quantum order.</span>
<span class="attribution"><span class="source">Nandini Trivedi</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Physicists have known about quantum order in superfluids and magnets in three dimensions since the middle of the last century. We understand that the order is lost at a critical temperature due to thermal fluctuations. But in two dimensions the situation is different. Early theoretical work showed that thermal fluctuations would destroy the quantum order even at very low temperatures. </p>
<p>What Thouless, Haldane and Kosterlitz addressed were two important questions: What is the nature of the quantum ordered state of superfluids, superconductors and magnets in low dimensions? What is the nature of the phase transition from the ordered to the disordered state in two dimensions? </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/140892/original/image-20161007-21421-1gllk4k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/140892/original/image-20161007-21421-1gllk4k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/140892/original/image-20161007-21421-1gllk4k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=317&fit=crop&dpr=1 600w, https://images.theconversation.com/files/140892/original/image-20161007-21421-1gllk4k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=317&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/140892/original/image-20161007-21421-1gllk4k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=317&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/140892/original/image-20161007-21421-1gllk4k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=399&fit=crop&dpr=1 754w, https://images.theconversation.com/files/140892/original/image-20161007-21421-1gllk4k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=399&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/140892/original/image-20161007-21421-1gllk4k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=399&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 whirl of a topological defect, a vortex or an anti-vortex, can be felt no matter how far you go from the eye of the storm.</span>
<span class="attribution"><span class="source">Nandini Trivedi</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Thinking about defects</h2>
<p>Kosterlitz and Thouless’s innovation was to show that topological defects – vortex and anti-vortex whirls and swirls – are crucial to understand the magnetic and superfluid states of matter in two dimensions. These defects are not just local perturbations in the quantum order; they produce a winding or circulation as one goes around it. The vorticity, which measures how many times one winds around, is measured in integer units of the circulation.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/140906/original/image-20161007-21430-w2slpc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/140906/original/image-20161007-21430-w2slpc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/140906/original/image-20161007-21430-w2slpc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=248&fit=crop&dpr=1 600w, https://images.theconversation.com/files/140906/original/image-20161007-21430-w2slpc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=248&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/140906/original/image-20161007-21430-w2slpc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=248&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/140906/original/image-20161007-21430-w2slpc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=312&fit=crop&dpr=1 754w, https://images.theconversation.com/files/140906/original/image-20161007-21430-w2slpc.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=312&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/140906/original/image-20161007-21430-w2slpc.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=312&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">On the left, a vortex is bound up with an anti-vortex. On the right, more and more defects unbind upon increasing the temperature, and the material enters a disordered state.</span>
<span class="attribution"><span class="source">Nandini Trivedi</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Kosterlitz and Thouless showed that at low temperatures, a vortex is bound up with an anti-vortex so the order survives. As the temperature increases, these defects unbind and grow in number and that drives a transition from an ordered to a disordered state. </p>
<p>It’s been possible to visualize the vortices in cold atomic gases that Kosterlitz and Thouless originally proposed, <a href="http://doi.org/10.1038/nature04851">bringing to life the topological defects they theoretically proposed</a>. In my own research, <a href="http://doi.org/10.1038/nphys983">we’ve been able to extend these ideas</a> to quantum phase transitions driven by increasing interactions between the atoms rather than by temperature fluctuations.</p>
<h2>Figuring out step-wise changes in materials</h2>
<p>The second part of the Nobel Prize went to Thouless and Haldane for discovering new topological states of matter and for showing how to describe them in terms of topological invariants. </p>
<p>Physicists knew about the existence of a phenomenon called the quantum Hall effect, first observed in two dimensional electrons in semiconductors. The Hall conductance, which is the ratio of the transverse voltage and the current, was observed to change in very precise integer steps as the magnetic field was increased. This was puzzling because real materials are disordered and messy. How could something so precise be seen in experiments?</p>
<p>It turns out that the current flows only in narrow channels at the edges and not within the bulk of the material. The number of channels is controlled by the magnetic field. Every time an additional channel or lane gets added to the highway, the conductance increase by a very precise integer step, with a precision of one part in billion. </p>
<p>Thouless’ insight was to show that the flow of electrons at the boundaries has a topological character: the flow is not perturbed by defects – the current just bends around them and continues with its onward flow. This is similar to strong water flow in a river that bends around boulders.</p>
<p>Thouless figured out that here was a new kind of order, represented by a topological index that counts the number of edge states at the boundary. That’s just like how the number of holes (zero in a sphere, one in a doughnut, two in glasses, three in a pretzel) define the topology of a shape and the robustness of the shape so long as it is deformed smoothly and the number of holes remains unchanged. </p>
<h2>Global, not local, properties</h2>
<p>Interacting topological states are even more remarkable and truly bizarre in that they harbor fractionalized excitations. We’re used to thinking of an electron, for instance, with its charge of e as being indivisible. But, in the presence of strong interactions, as in the fractional quantum Hall experiments, the electron indeed fractionalizes into three pieces each carrying a third of a charge! </p>
<p>Haldane discovered a whole new paradigm: in a chain of spins with one unit of magnetic moment, the edge spins are fractionalized into units of one-half. Remarkably, the global topological properties of the chain completely determine the unusual behavior at the edges. Haldane’s remarkable predictions have been verified by experiments on solid state materials containing one-dimensional chains of magnetic ions.</p>
<p>Topological states are new additions to the list of phases of matter, such as, solid, liquid, gas, and even superfluids, superconductors and magnets. The laureates’ ideas have opened the floodgates for prizeworthy predictions and observations of topological insulators and topological superconductors. The <a href="http://doi.org/10.1038/nature13915">cold atomic gases present opportunities</a> beyond what can be achieved in materials because of the greater variety of atomic spin states and highly tunable interactions. Beyond the rewards of untangling fascinating aspects of our physical world, this research opens the possibility of using topologically protected states for quantum computing.</p><img src="https://counter.theconversation.com/content/66543/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nandini Trivedi 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>Forget solid, liquid, gas. This research used advanced math to theorize about topological phases of matter. And over the years experiments with matter and cold atoms have been validating the ideas.Nandini Trivedi, Professor of Physics, The Ohio State UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/366152015-01-27T05:59:51Z2015-01-27T05:59:51ZIs glass a solid or a liquid?<figure><img src="https://images.theconversation.com/files/69800/original/image-20150122-12082-1nl2jww.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Make up your mind, glass.</span> <span class="attribution"><span class="source">jurra8</span></span></figcaption></figure><p>Before Pilkingtons <a href="http://www.pilkington.com/pilkington-information/about+pilkington/education/sir+alastair+pilkington.htm">invented</a> plate glass in the mid-19th century, flat panes could not be made. Old windows are uneven.</p>
<p>Some once thought this was because glass is a liquid that flows down slowly over the centuries. That’s wrong. But although glass does not move on timescales as “short” as centuries, the true nature of glass, whether it is truly solid or a very viscous liquid is something we do not know.</p>
<p>Solving the challenge, determining whether glass can become truly solid, requires identifying a clear transformation, the equivalent of liquid water freezing to ice as temperature changes. Of course glass can be found in a liquid state, but does it become a true solid once it cools down? </p>
<p>The problem with glass-forming materials – which include plastics, alloys and ceramics in addition to everyday “glass” – is that there is no obvious transformation. On cooling we cannot definitively say yet that glass has become a solid.</p>
<h1>The glass transition</h1>
<p>Glass is so poorly understood that if a transition occurs it is far from universally accepted. It is said that “there are more theories of the glass transition than there are theorists who propose them”. </p>
<p>During a scientific revolution, many interpretations of what is happening compete before one interpretation becomes accepted by the scientific community as being correct. In this respect, our understanding of what happens when a liquid is cooled, whether it forms an “ideal glass”, is undergoing a modern-day scientific revolution.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/69788/original/image-20150122-12117-16wjxav.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/69788/original/image-20150122-12117-16wjxav.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=635&fit=crop&dpr=1 600w, https://images.theconversation.com/files/69788/original/image-20150122-12117-16wjxav.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=635&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/69788/original/image-20150122-12117-16wjxav.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=635&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/69788/original/image-20150122-12117-16wjxav.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=797&fit=crop&dpr=1 754w, https://images.theconversation.com/files/69788/original/image-20150122-12117-16wjxav.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=797&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/69788/original/image-20150122-12117-16wjxav.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=797&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Molecules in a viscous liquid. Blue regions are solid-like, green, yellow and red atoms are more liquid-like.</span>
<span class="attribution"><span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>The reason it is hard to observe a liquid transforming to a perfectly solid “ideal glass” is that to do so we would have to wait an extraordinarily long time (much longer than centuries) because the process is very slow. But the behaviour of viscous liquids is more surprising than simply having to wait a long time. </p>
<p>If we look at the microscopic behaviour of small groups of molecules, viscous liquids seem to find it hard to make up their mind whether to be solid or liquid. Regions of a few tens of molecules in size are liquid-like, others are solid-like. Over time these regions change between being solid-like or liquid-like state. This is totally different to water freezing, when all the molecules together decide to form a solid.</p>
<p>We tried to answer this question and found a new way to understand how molecules behave in these small regions in a <a href="http://www.eurekalert.org/pub_releases/2015-01/uob-iga012115.php">viscous liquid</a>. We used information theory originally developed in Bletchley Park for code-breaking to find out how molecules in the solid-like and liquid-like regions communicate with one another. It involves the use of calculating multiple permutations and combinations of interactions between atoms, much like the permutations and combinations needed to break the Enigma code.</p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/69791/original/image-20150122-12082-1r58v2x.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/69791/original/image-20150122-12082-1r58v2x.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/69791/original/image-20150122-12082-1r58v2x.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/69791/original/image-20150122-12082-1r58v2x.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/69791/original/image-20150122-12082-1r58v2x.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/69791/original/image-20150122-12082-1r58v2x.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/69791/original/image-20150122-12082-1r58v2x.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Communication in a viscous liquid. We have shown that the red molecule communicates only with a select group of neighbours (shown in blue). Other transparent molecules are</span>
<span class="attribution"><span class="license">Author provided</span></span>
</figcaption>
</figure>
<p>Each molecule “talks to” a select group of neighbours and that group of molecules is either solid-like or liquid-like. Simultaneously the groups of solid-like molecules organise into arrangements of icosahedra – one of the five Platonic solids – predicted by Charles Frank, also at the University of Bristol, back in 1952. Meanwhile the groups of liquid-like molecules are less organised, often making squares and triangular prisms which aren’t as rigid (solid-like) as icosahedra.</p>
<p>Our work shows that the numbers of solid-like molecules in icosahedra increase as the material becomes more viscous, and the size of these regions of molecules organised into icosahedra becomes larger and larger. Eventually all the molecules are part of these solid regions and the material becomes an ideal (perfectly solid) glass. This supports the idea of the existence of an ideal glass, taking us closer to resolving the scientific revolution that is the glass transition.</p>
<p>Understanding the behaviour of glass-forming materials and whether there is a true solid glass is important in the development of metallic glasses. The mechanical properties, such as stiffness, of these glasses are superior to normal metals. Another important class of glass formers are chalcogenide materials, which are the basis of optical storage and are an important future technology for high-performance non-volatile hard drives.</p><img src="https://counter.theconversation.com/content/36615/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Paddy Royall receives funding from the Royal Society, European Research Council and Engineering and Physical Sciences Research Council.</span></em></p>Before Pilkingtons invented plate glass in the mid-19th century, flat panes could not be made. Old windows are uneven. Some once thought this was because glass is a liquid that flows down slowly over the…Paddy Royall, Lecturer, University of BristolLicensed as Creative Commons – attribution, no derivatives.