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Boiling water without bubbles – that’s just our cup of tea

Imagine a specially-engineered surface that could allow liquids to boil without bubbling. This sounds counter-intuitive and, in a way, it is. But consider the following. When a small drop of water is dropped…

Hubble, bubble, boil and … no, wait, hold the bubbles. Velo Steve

Imagine a specially-engineered surface that could allow liquids to boil without bubbling. This sounds counter-intuitive and, in a way, it is. But consider the following.

When a small drop of water is dropped onto a very hot frying pan, it skitters around and takes up to a minute to evaporate. A video of this phenomenon is embedded below.

On initial contact, the hot surface vaporises part of the drop and creates an insulating vapour layer between the drop and the hot surface, much like the air gap in a double-glazed window. This vapour layer can only be sustained if the hot surface is above the so-called Leidenfrost temperature. It also acts as an efficient lubricant and can reduce the drag on a hot sphere travelling through water by up to 85%.

The Leidenfrost vapour layer also plays an important part in boiling and cooling. If in place of small drops of water in a hot frying pan, we have a hot kettle filled with water, the Leidenfrost vapour layer will collapse when the kettle cools below the Leidenfrost temperature, resulting in an explosion of vapour bubbles as the water makes direct contact with the (still) hot surface.

Until recently, this rather violent explosive ending for vapour, reminiscent of the death throes of a star, was thought to be inevitable when a hot surface in contact with water cooled below the Leidenfrost temperature.

Everything changes

In a report published yesterday by my colleagues and I in the journal Nature, it was found that by modifying the heating surface to give it nano-scale roughness and a coating to render it highly water repellent – or super-hydrophobic – the Leidenfrost vapour layer can be sustained at all surface temperatures, thereby eliminating the vapour explosion.

A video illustrating our work can be watched below:

The research was undertaken by members of the University of Melbourne, the King Abdul University of Science and Technology, Saudia Arabia, and Northwestern University in the US.

It might sound a little complicated but the way it works is simplicity itself. The rough, water-repellent surface acts like the fakir bed of nails so that the water rests on the peaks of the rough nano-scale surface layer and makes very little solid-liquid contact.

We’ve nailed it. minipixel

With the right combination of roughness and chemical modification, the bubble explosion resulting from collapse of the Leidenfrost vapour layer can be suppressed.

These results may be useful in applications in which “quiet” boiling is required – whether it be for a non-intrusive kettle or boilers that generate minimal vibrations and noise.

The general principle this work reveals is that, in any process in which heat is transferred between a solid and a liquid during heating, cooling or freezing – or more generally a change of phase – the surface properties of the solid heat source or sink should be factored into the design.

Examples of potential areas of application may be controlling ice formation on the ailerons of aircrafts, the fogging and frosting of mirrors and windows and in refrigeration, in which the liquid coolant is a refrigerant, not water.

If you thought liquids had to come to an explosively violent end, we may just have burst your bubble.

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20 Comments sorted by

  1. Ian Donald Lowe

    Seeker of Truth

    Derek, this is certainly interesting work and I can see the potential, especially in refrigeration and perhaps even in steam driven turbines, or somewhere down the track, space flight. I personally would need to know more about the nano-coating substance and it's properties before I would even consider buying any appliance or vessel that would be used to heat water or food for consumption.

    There has been much debate on a link between alluminium and neurological disorders such as alzheimers and there has been another debate about the link between certain plastics and genetic deterioration (cancers).
    I can understand why you would be loathe to release the specific information I would need to feel comfortable with a kettle with this type of nano-coating at this point, so all I can say is good luck with your further research but I prefer my food and beverages to be as un-tainted as possible by any chemical substances. I hope you can understand that.

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    1. Aidan Brock

      PhD Student (Inorganic Chemistry)

      In reply to Ian Donald Lowe

      "...I prefer my food and beverages to be as un-tainted as possible by any chemical substances"

      So what do you eat, then? All foods and beverages contain chemical substances. Indeed, by definition, all matter is a chemical substance. Water, an incredible variety of proteins, cellulose and other carbohydrate polymers.

      By your statement, I assume you mean 'introduced' or 'not natural' chemicals such as nanoparticles. Not all nanoparticles are dangerous. In fact, some have recently been found to…

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    2. Ian Donald Lowe

      Seeker of Truth

      In reply to Aidan Brock

      "With regards to toxicity of the superhydrophobic coating, there are several polymers with low toxicity, for example, perfluoropolyether [2]."

      Low toxicity is still toxic.

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    3. Matthew Wyres

      Mechanical Engineer

      In reply to Ian Donald Lowe

      Just a side point; the claimed link between aluminium and neurological diseases has been disproven for a while, but still persists in culture and in some forms of media (I suspect as the idea of Aluminium toxicity has been around for a while and will take a while to disappear completely).

      On the other points; a nanotextured surface does not necessarily mean nanoparticles, but besides that many nanoparticles have been shown to be non toxic so far although definitely much more work is required before exposing people to them commercially. Also, 'low toxicity' generally is only toxic in large quantities, so ingestion in low quantities is non-toxic.

      As a comparison; mercury is toxic and if you eat fish, then you ingest mercury but usually not in large enough quantities to cause toxicity.

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    4. Ian Donald Lowe

      Seeker of Truth

      In reply to Matthew Wyres

      Mercury is an accumulative poison, so small doses over an extended period of time can be deadly.
      Also, seeing as there are no medical tests for neurological disorders, other than brain scans for signs of tumours, how can the link between alluminium and neurological disorders be disproven. Psychiatry is only a psuedo-science that bases it's diagnoses on symptoms and as such, there can be no scientific proof of anything relating to mental illnesses. Yes, you can do studies but those are just statistical and there are lies, damned lies and statistics.

      It's a pity the author didn't bother answering my concerns, so I guess I will never know.

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    5. Yoron Hamber

      Thinking

      In reply to Aidan Brock

      That is true, but it is also true that we don't know what nano technology may expose us to. And there is research going on, and there will be regulations as the industry starts to embrace it. So there is a point to what Aidan writes, it surprises me that he didn't get a more expansed reply. As for all substances being toxic, true maybe? Are you considering decay processes there? But we've evolutionary evolved with the food we eat, that's a quite different truth, from assuming that because everything can be broken down into nano scale, everything is as good for us humans, as well as all other species we share this earth with.

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  2. Seán McNally

    Market and Social Researcher at eris strategy

    Derek, congratulations to the team for undertaking the work and getting published in Nature. While this is not my area of expertise, it looks on the surface to have fare reaching applications to places where improving fluid movement delivers benefits. It would be interesting to know if by not having the bubbles aspects of food processing are improved.

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  3. Mark Amey

    logged in via Facebook

    'Boiling water without bubbles – that’s just our cup of tea'

    'Tell 'em they're dreamin' was my response to this, but no, read on...bloody brilliant. Prof Chan has probably started a revolution, not only in cooking appliances (which is probably a trivial application) but, as others have pointed out, just about any application where liquids interact with solid surfaces!

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    1. Norm Stone

      Farmer

      In reply to Mark Amey

      Cooking is trivial is it? Better stop eating or modify your diet then. Have some raw eggs with perhaps some ground wheat for brekkie!

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    2. Mark Amey

      logged in via Facebook

      In reply to Norm Stone

      No, Norm, cooking isn't trivial, but 'll bet there are plenty of applications beyond 'silent cooking'!

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  4. William Ferguson

    Software Developer

    Lots of applications .. but personally if this can deliver a quiet kettle it's enough.

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  5. Jonathan Maddox
    Jonathan Maddox is a Friend of The Conversation.

    Software Engineer

    I may be being really obtuse here, but isn't the Leidenfrost vapour layer an insulator, allowing small quantities of water to boil while the main body of the drop remains below boiling point?

    And in a water-boiling application don't you generally want to *maximise* heat transfer and therefore the contact area between the heated vessel and the water inside it, and wouldn't this surface *minimises* it? You might get a quiet kettle but intuitively at least, I'd expect the technique to maximise steam production at the expense of the quantity of boiling-hot liquid water delivered at the end of the process. So is it about tea-making at all?

    I know water can be heated directly without contact, ie. with microwaves, but that wasn't mentioned in the article.

    If someone can point out if, or where, my mental model of this technique is wrong, I'd like to know :-)

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  6. rachel polanskis

    Antifascist

    An exaggerated version of the effect is when you heat water in a microwave oven, it does not boil,
    but as soon as you add something like instant coffee, it boils over in a rush. This is because the water molecules need a nucleus of something to form a bubble around. As soon as the coffee is added, it provides this medium, the same with the inside of a kettle, as the bubbles form on the walls and element which also makes the rushing noise.
    I think this technology has implications for aerodynamics and perhaps competitive swimwear.
    It might also fix certain engineering problems,
    like moving gas or oil more efficiently....

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    1. Mark Amey

      logged in via Facebook

      In reply to rachel polanskis

      I suspect that the water boils when one adds instant coffee because, once the microwave is turned off, the water drops to just below boiling point, and the latent heat of solvation of the coffee granules, when added, although small, is sufficient to increase the water temp back up to boiling point.

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    2. Matthew Rowles

      logged in via Facebook

      In reply to Mark Amey

      It's most definitely providing a nucleation point. I've done this in chemistry laboratories when adding boiling chips to a super-heated liquid; you get explosive boiling.

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  7. Kevin Orrman-Rossiter

    Research Partnerships Officer at University of Melbourne

    Great piece of research - congratulations to Derek Chan and co-researchers. I trust there are some willing entrepreneurs chasing this research up to develop some innovative technologies.

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