Why does superfluid helium creep up surfaces?

Helium, which turns into liquid at about 4.2 Kelvin, can be held in a container like a beaker due to gravity. But when it is cooled further to below approximately 2 Kelvin, it creeps up the surface of the beaker and leak. At this temperature, liquid helium is called as superfluid due to its odd properties. For example, the liquid's viscosity becomes nearly zero. Because of that, the fluid can flow very easily even as a result of the smallest pressure. On one hand, a thin liquid helium film will appear as the liquid wet the surface of the beaker. On the other hand, liquid helium has smaller dielectric permittivity than any other medium (except vapour), which results in a negative Hamaker constant and a repulsive van der Waals force across the film. This will act to thicken the film and make the liquid helium flow from the bottom of the beaker to its surface and thus leak. 

How to Supercool Water

One of the interested facts about water is that it does not always freeze and 273.15 K, as it normally does. Actually pure water, water that has no impurities and free of nucleation sites, can stay in liquid form up to 224.8 K.

When the purer water is supercooled it is very easy to freeze it, because any iteraction with the water molecules in that state can result in the fomration of nuclation sites, that will instantly result in the freezing of the water.

This video is a simple and nice representation on how to supercool water and then freeze it instantly!


MicroFluidics and Non-Equilibrium Gas Flows Conference 2018

The 5th European Conference on Microfluidcs and the 3rd European Confenrencen on Non-Equilibrium Gas Flows (NEGF) had been held on 28th of Febraury - 2nd March at the University of Strasboug, France. Two members from our MNF Group, Prof. David Emerson and I, and one of Prof. Yonghao Zhang's Postdocs, Dr. Minh-Tuan Ho attended this joint conference, and we all gave 15 minutes long presentations. David also chaired my session on the second day afternoon of the conference. Lots of simulations and modelling had been presented in the conferene on NEGF, including the conventional direct simulation Monte Carlo (DSMC) solver for rarefied gases and molecular dyanmics (MD) simulations such as CH4/CO2 molecules hitting graphitic walls. There were plenty of experiments of microfluidics, including the vapour adsorption phenomena onto liquid desiccant droplets (the presnter who is a first-yesr PhD student in Japan was awarded the first prize for the best presentiaons) and microchannel flows such as formation dynamics of ferrofluid droplets in a T-junction. The psoters were also very exciting and many beatiful works had been displayed during the coffee break. The local French committee did a really good job and were very helpful. The next conference will be held in Germany in 2020 and all are welcome to present and share their interesting research. (I got the soft copies of abstracts of this conference and I am happy to send them to you if you are interested)



Evaporation-driven vapour micro flows: analytical solutions from moment methods

Our recent research article has recently been accepted in Journal of Fluid Mechanics. The general interest of this study was to explore boundary value problems for moderately rarefied gas flows, with an emphasis on evaporation from nanostructures. In this article we developed macroscopic models based on R26-moment equations to describe the transport process near the phase boundary between a liquid and its rarefied vapour due to evaporation and condensation. For evaporation from a spherical droplet, analytic solutions were obtained to the linearized equations from the Navier Stokes & Fourier (NSF), regularised 13-moment (R13) and regularised 26-moment (R26) frameworks. Results are shown to approach computational solutions to the Boltzmann equation as the number of moments are increased, with good agreement for Knudsen number smaller than 1.


Knudsen layer functions in temperature-driven flow (far field temperature is twice of the temperature in liquid) are shown in Figure above. Curves of the normalised pressure (left) and the temperature defect (right) are plotted against scaled distance from the interface, for the NSF, R13 and R26 theories with complete evaporation and compared with the solutions of the linearized Boltzmann equation (denoted by symbols).   

Our results indicate that the R26 equations with evaporation boundary conditions yield an excellent quantitative description in all cases, which are not matched by the NSF or R13 theories. The R26 system provides three exponential functions to form the Knudsen layer, thus capturing the more complex behaviour in Knudsen layer.

Drops on slip-pery surfaces

A recurrent theme on this blog has been posts describing talks presented at conferences. In keeping with that tradition, I’ll use this post to briefly introduce two talks from a recent-ish conference that I had the privilege of attending. Both talks relied on the use of what are known as slippery-liquid-infused-porous (SLIP) surfaces, or lubricant-impregnated surfaces. According to the folks who make them, SLIP surfaces 'combine a lubricated film on a porous solid material to create low-cost surfaces that exhibit ultra-liquid repellency, self-healing, optical transparency, pressure stability, and self-cleaning'. While this sounds impressive and worthy of further unpacking, I'll leave the discussion on the science behind such specialised surfaces for a future blogpost, and keep this one focused on the fluid dyamics instead, i.e. the drops. 


Making a SLIP surface. Image credit: Wyss institute (Original here)


The first talk, by Zuzana Brabcova, discussed her recent results published in these two papers (one, two). Basically, there exist two popular techniques to electrically manipulate drops on surfaces. The first is electrowetting, where the ions in a conducting liquid droplet are transported by the electric field, resulting in an electrophoretic interfacial force. The other one is dielectrowetting, where the electric field acts on dipoles (in an effectively dielectric liquid droplet) at the solid-liquid interface, resulting in a dielectric interfacial force. One of the problems when using either mechanism is the observed contact-line hysteresis, which prevents a smooth transition from wetting to dewetting. This talk demonstrated that, by using a SLIP surface combined with custom spiral electrodes, the hysteresis for electrowetting and dielectrowetting could be completely removed. By modifying the applied signal, near-axisymmetric wetting was also realised, resulting in the formation of a circular thin film on demand. 


The second talk, by Jian Guan, looked at translating drops on SLIP surfaces. To induce controlled drop motion on a SLIP surface, a patterned surface (before the lubricant layer is applied) was used, resulting in an uneven lubricant distribution on the surface. For a V-shaped channel, droplets moved towards the regions of higher lubricant deposition to minimise surface energy; these regions were invariably near the edges of the channel. The size of the droplet determined its net motion. For small droplets, either side of the channel was the energetically-preferred destination, while for larger droplets which straddled both ends of the channel, motion into along the channel was observed. Thus the motion and final equilibrium position of the droplet could be determined in advance using patterned SLIP surfaces. These results are discussed in further detail in the author's recent paper on the topic (link).

The Mpemba effect

The Mpemba Effect is a counter-intuitive phenomenon whereby, under certain circumstances, warm water can freeze faster than cold water. Although this phenomenon was known for a long time by many philosophers and scientist, including Aristotle, Rene' Descartes and Francis Bacon, it was brought to the attention of the scientific community by a Tanzanian schoolboy, Erasto Mpemba. In 1963, during a school project, he noticed that an ice-cream mixture, previously heated, froze more rapidly than one that was cold. He was so intrigued by this phenomenon that he continued experiments until, a few years later, he went on to work with a physics professor, Denis Osborne, and together they published a paper in the journal Physics Education in 1969.

A public competition announced by the Royal Society of Chemistry in 2012 to explain the Mpemba effect renewed the interest in this phenomenon.

The above video provides a very clear overview of the various explanations that have been put forward. These include evaporation (the evaporation rate of warm water is higher and, consequently, there is a smaller amount of water to cool down), dissolved gases (the warm water contains less dissolved gas, which, apparently, hinders the ability of water to conduct heat), supercooling (the warm water contains more nucleation sites which makes it unlikely the need to get temperatures less than zero degrees Celsius for freezing), convection (the convection currents in the warm water are stronger and increase heat dissipation).

None of these explanations is entirely convincing. A recent study has even concluded that the Mpemba effect is just an experimental artifact due to measurements' inaccuracies. On the other hand, another recent study has shown, by theory and simulation, that the Mpemba effect and its reverse (a cooler sample may heat faster than a hotter sample) may occur in uniform granular fluids composed of inelastic particles and an explaination can be given based on the kurtosis of particle velocity distribution function.

It's surprising that something as apparently simple as the freezing of water is by contrast still poorly understood.

Jumping droplets for cooling applications

One of the "hottest" applications of what I am doing at The University of Edinburgh as a part of my PhD is the "coalescence induced jumping of droplets for cooling purposes over specially treated surfaces". By "specially treated surfaces", I mean super-hydrophobic surfaces upon which droplets (mainly water) sit like a ball sits on a floor.

When two such neighboring droplets coalesce, the final droplet will have a smaller surface area than the combined areas of the first two. This reduction in the area supplies some energy for the final droplet to jump off the surface. That's the physics (certainly not all!) behind it.

I know what the applications of this interesting phenomena are, but it's really difficult to explain them just using words to friends, parents etc. That's when I found this video made by a research team from Duke University.









Apparently, insects like Cicada are already experts in this field. One of the main advantages of this mechanism is that you do not need any pump to make the​water reach a height above. This potentially avoids the involvement of any moving parts in the cooling mechanism and we do not have to worry about any frictional losses. Further details can be found at http://pratt.duke.edu/news/cooling-droplets.

Even more conferences!

There have been a few posts here recently about conferences and other meetings. I have been to four over the last several months, so it is not hard for me to continue this fine tradition.

The first one, back in September, was actually not a regular conference, but rather a Summer School, on Complex Motion in Fluids, in Cambridge. As is often the case with such events, it was geared mostly towards PhD students (so I felt a bit old at times!), we lived in a student residence (in this case, one of Cambridge colleges) and the speakers, instead of regular talks, tried to give more pedagogical overviews of their research or a whole research area. This particular summer school is a yearly event (so it has developed some strange traditions, like singing a song together) and a collaboration between four universities, one each in France, Denmark, the Netherlands, and the UK. There were nine principal speakers (plus a keynote talk by Keith Moffatt, a local fluid dynamics celebrity) and most gave two lectures. The range of topics was quite broad --- here are a few examples. Henrik Bruus (notable for being the author of books on topics as different as microfluidics and many-body quantum theory of solids) talked about flows created by sound (acoustofluidics) and how they can be used to sort and hold particles. Detlef Lohse (the head of a huge group at Twente) discussed drop impact on a heated surface (a topic of particular interest to me, as it seems a natural application of the computational approach I am developing), and in his second lecture, a variation of the ouzo effect (mentioned on this blog some time ago), where a drop consisting of a mixture of water, alcohol and oil evaporates, a multistage phenomenon producing fascinatingly complex flows. Dominic Vella talked about elastocapillarity, in particular, wrinkling elastic sheets on liquid surfaces and a surprisingly complex problem of wet brush bristles sticking to each other. Denis Bartolo's lectures were about a system of active colloids and its hydrodynamic description and Roberto di Leonardo talked about microswimmers and, in particular, how bacteria can drive a motor and how this can be controlled by light. Jacques Magnaudet discussed flows created when a disc falls in a liquid or a particle crosses an interface. Anne-Laure Biance talked about various electrokinetic effects, in particular, how an electric field can stabilise a foam. In addition, nearly all of more than 100 attendees either gave a short talk or had a poster, which broadened the range of topics even more. I had a poster as well; in fact, it was the first "proper" A0 poster I've ever done, as in the past I would just print out a few A4 sheets, perhaps gluing them together (or not). All in all, it was good fun. The only problem was, the college where we were based was literally in the middle of nowhere and too far from the centre of the city, so I only managed one trip to the centre and with the traditional Warwick fluids walking trip immediately afterwards there was no chance to stay longer. So I had to run an orienteering race in Cambridge in October as an excuse for another visit to the city.

The last discussion at the Cambridge Summer School before departure

The second conference was the APS DFD meeting in Denver. Juan has already described it very nicely in his post (which even has my picture!), but I'd like to add my perspective from the point of view of someone who attended around a dozen APS meetings in the past - just not the DFD ones! As a condensed matter physicist, the meeting I would normally go to is the APS March Meeting dedicated mostly to condensed matter and biophysics. The basic structure is the same - most talks are 10 minutes long, with 2 additional minutes for questions. However, there are subtle but crucial differences. First, there is a one-minute break between the 12-minute periods in the DFD meeting, but not in the March meeting. Second, during a talk the remaining time is shown on a big screen in the DFD meeting, but in the March meeting there are only sound signals to warn you two minutes before the end. The result is that the DFD meeting is run much more smoothly and strictly on schedule, while during the March meeting running behind schedule is very common. Also, the one-minute break is enough time (though just barely) to move between different sessions. On the other hand, the thing that I like more about the March Meeting is that there are many more longer (30-minute) invited talks - most sessions have one or two and there are many sessions consisting of just invited talks. Longer talks are easier to understand and they are a great way to learn about a new area. I must say, though, that many 10-minute talks at the DFD meeting were quite understandable despite the limited amount of time the speakers had, so this was not as much of a problem as I feared. As for the content, my choice of talks to attend, obviously, reflected my interests. So I went to a lot of drop impact talks, both experimental and computational, including drops falling on liquid films (hot and cold, still and flowing, miscible and immiscible), on inclined, moving and "hairy"  surfaces, on granular media, under reduced ambient pressure, superfluid drops and even drops shot with a bullet! One session mostly consisted of talks (many from the Bush group at MIT) on drops "walking" on a vibrating fluid bath, a system that was suggested a while ago as having some similarities to quantum systems. There were also several sessions on wetting and moving contact lines (James gave a talk in one). Many talks were on instabilities of liquid jets and films and I went to a few, as it is another area of research in which our group is involved. Examples of systems that the speakers considered are a ferrofluid jet on a wire through which an electric current is driven (this talk was later repeated at the Oxford workshop mentioned below), the effect of surface viscosity on the Rayleigh-Plateau instability (doubly interesting, as Jesse uses the surface viscosity concept to model particle-laden drops) and stability of metal films subjected to laser irradiation. Among other bits and pieces I found interesting was another talk on surface rheology (with particles) and work from Snoeijer's group on the distinction between surface tension and surface energy for wetting of stretched solids. Tourism-wise, as it was my fourth visit to Denver, there was not much left to see, but I did go to the local museum of nature and science with some nice dioramas and interactive exhibits.

Some time ago, Livio wrote a post about a meeting of the Multiscale and Non-continuum Flows Special Interest Group (SIG). It is not the only fluids SIG  - in fact, there are 41 of them! Another one of interest that James is a member of is on Drop Dynamics. It is chaired by two brothers, Alfonso and Rafael Castrejón-Pita, and both meetings held so far were in Oxford. The first one was in April and consisted of talks by the group leaders; for the second one, the organisers gave the opportunity to speak to students and postdocs. So I gave a talk there as well. I don't talk at small conferences very often and I must say that it is a nice experience: you don't need to introduce yourself and people often approach you themselves to chat and so even someone as socially awkward as me was able to talk to a few people. As for the other talks, perhaps the most unusual one (and only tangentially related to the topic of the workshop) was on electrospinning of polymers used to create ... a model of white matter of the brain. Closer to the topic of drops, there were a couple of talks about drops moving on oil-impregnated surfaces providing very low contact angle hysteresis and therefore friction; there were also a couple of drop impact talks, one about simulating drops hitting a surface at 100 m/s (!) and the other about impact on hydrophobic textiles (think fancy umbrellas). Ciro Semprebon talked about Lattice Boltzmann simulations, in particular, of drop-drop collisions; obviously, he was not able to resolve the air film between the drops, so only drops of two immiscible fluids could be simulated and he was understandably impressed by our simulations of identical drops. There was also a nice talk by Chuan Gu on simulating particle-covered drops; it is of obvious relevance to the problem Jesse is working on and we have decided to invite Chuan's boss, Lorenzo Botto, to Warwick --- he will give a talk at our fluids seminar in a couple of weeks.

Finally, the Technical Workshop in Cheshire ... but as Stephen and Jason wrote about it and my post is already pretty long, I'll just say that it was fun and gave me an opportunity to visit Liverpool afterwards and will finally shut up!

Happy New Year! Now how to deal with the snow?

Happy New Year, all! While the Hogmanay hangovers are beginning to wear off, we approach that time of year when the news headlines are dominated with “disruption”, “travel chaos”, and “winter storms”. I personally love snow, but when it comes to freezing temperatures and delays I definitely understand why some people don’t.

Airport staff possibly have the greatest reason to dislike snow. Aside from general runway clearing, deicing airplanes is an expensive, time-consuming, yet very necessary task. Ice accumulation on the wings of an aircraft increases weight which reduces fuel efficiency, but crucially also adversely affects airflow over the plane resulting in poor lift. Current deicing approaches are to spray airplanes with a heated glycol/water mix to melt existing ice and snow (glycol decreases the freezing temperature of water). A second step is also sometimes to spray the aircraft with a thicker, more concentrated glycol/water mix which prevents further ice forming on the aircraft body. This secondary deicing liquid slides off the plane during take-off so needs to be reapplied for every flight.

Part of our research at Micro & Nano Flows for Engineering is to investigate the onset of ice formation and how we can engineer materials which prevent the formation of ice on their surfaces, with the aim of eliminating the need for manual deicing processes. Applications for these technologies extend to almost all modes of transportation, such as railways, ships and roadways.

Lastly, in defence of winter I offer a photo of Edinburgh in the snow taken from Arthur’s seat during the holidays, as if you needed any more motivation to join us!

An end-of-year celebration of research

For me, the highlights of each research year are the Steering & Impact Committee meetings (where we bring together our industrial partners) and the Technical Workshops (where we share our progress between our institutions). On Monday and Tuesday of this week we had our regular end-of-year research extravaganza, where we not only have a Steering & Impact meeting, but the following day our Technical Workshop which we open up to our other close collaborators at Strathclyde, Glasgow and Heriot-Watt universities. This year we met in the beautiful surroundings of Inglewood Manor Hotel, near Chester.

Above is a photo, taken on the steps of the hotel, of our industrial partner representatives from Jaguar Land Rover, the European Space Agency, TotalSim Ltd, Waters UK, Nokia Bell Labs (Ireland), TH Collaborative Innovation, and Akzo Nobel Coatings International (not shown). We were able to share with our partners our progress made in the last six months, introduce and hear from our researchers, and discuss mutual opportunities and any issues arising. Several of our PhD and postdoc projects have close industrial engagement so we had a lot to discuss, and the contributions of our industrial partners are invaluable! We finished the day with a lively discussion about the UK's new Industrial Strategy, followed by a convivial meal.

On Tuesday we had our Technical Workshop. Including our collaborators from Strathclyde University, I counted more than 40 people attending. Trying to fit so many talks from our researchers into the day, meant that we adopted an American Physical Society way of doing it, i.e. talks could only be 11 minutes, with 4 additional minutes for any questions from the audience.

The range and quality of the research talks by our researchers always impresses me, and this year was no exception. The excellent research results convinces me that future progress in engineering science is safe in the hands of the next generation! This year we were also pleased to welcome our new visiting scientist Prof Suman Chakraborty of the Indian Institute of Technology Kharagpur, who gave a well-received keynote talk on "Slippery flows: a molecular perspective". We are looking forward to welcoming Suman back to the UK for another visit to us later in 2018.

The day is a fantastic demonstration of the value of working with each other, sharing ideas, and helping each other. I encourage our people to take these opportunities to discuss their ideas, problems, hopes and fears with each other - we have seen time and time again that is the way to advance together. 

In the evening we gathered for another of our traditions - the end-of-year dinner. This is a great chance to relax together over a good meal, and the evening high-jinks often go on until the early hours of the next morning!


Latest News

Recent Publications

R Pillai, JD Berry, DJE Harvie, MR Davidson (2017) Electrophoretically mediated partial coalescence of a charged microdropChemical Engineering Science, 169: 273-283. (access here)

JF Xie, BY Cao (2017) Fast nanofluidics by travelling surface wavesMicrofluidics and Nanofluidics, 21: 111 (access here)

AP Gaylard, A Kabanovs, J Jilesen, K Kirwan, DA Lockerby (2017) Simulation of rear surface contamination for a simple bluff bodyJournal of Wind Engineering and Industrial Aerodynamics, 165: 13-22. (full paper here)