Shaken, not sprayed: A new way of heating liquids using vibrations

Jason, Matthew and I have had an article published in Physical Review Letters detailing our research in vibration-induced heating of water nanofilms. We were asked to provide a 200 word summary of the article, written in a manner accessible to the layperson. I found this to be a valuable exercise, as the required style of writing is completely different from that expected in a paper. I thought I'd post our final version here, as it may provide some value to future researchers within the group and elsewhere. For those interested, the actual paper is available at: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.104502
 
 
Shaken, not sprayed: A new way of heating liquids using vibrations 
 
If you vigorously shake a water droplet resting on a flat surface it can break up into a fine mist, similar to the liquid sprays from aerosol cans. However, we have shown, for the first time, that intense vibrations can also be used to boil water and other liquids. Using molecular simulations, we have demonstrated this effect in extremely thin liquid layers - some thousand times thinner than a human hair - resting on a vibrating surface. The vibrations also have to be very high frequency, around a million times quicker than the flapping of a hummingbird’s wings. Under these conditions the thin film of water boils, just due to the shaking – imagine a tiny vibrating kettle! This discovery could stimulate ideas for new nanotechnologies: vibrating nano-arrays may be able to prevent ice formation on airplane wings, cool the electronic circuits in our smartphones and laptops, and dry clothes quicker for lower electric bills. Thus, exploiting this new science of vibrations at the smallest scales could, literally, ‘shake things up’ in our everyday lives.
 

Turning Bubbles into Snow Globes

Although several mesmerizing videos of freezing soap bubbles out in the snow are available on YouTube, this phenomenon lacked scientific explanation. The following video explores some of the physics involved in freezing bubbles.

Researchers from Virginia Tech in Blacksburg investigated this phenomenon by depositing bubbles on a silicon substrate having temperatures between -10 and -40 degrees Celsius, with the surrounding air at room temperature. It was observed that the freeze front moves very slowly up the bubble, and in some cases even comes to a complete stop after reaching a critical height. The slow propagation of the freezing front is a result of the poor thermal conductivity of the thin soap film. The speed of propagation of the freeze front can be more readily observed in larger bubbles or at higher surface temperatures.

This delayed freezing consequently allows enough time for the frozen portion of the bubble surface to cool the air within the bubble, while the top part is still liquid. As a result, a pressure imbalance is developed that either collapses the top or causes the top to pop. When the freeze front manages to reach the top of the bubble, a section of the top may melt and slowly refreeze. This melting and refreezing cycle can take place several times in a single bubble.

The last part of the video shows freezing bubbles in a freezer, where the surrounding air was maintained at -20 degrees Celsius.  Under these conditions, the bubble freezes quickly and the ice grows radially from nucleation sites rather than perpendicular to the surface. This contrasts with the limited conductivity of bubbles deposited on a cold surface in room temperature.

Differences in surface tension (or Marangoni effects) create currents in soapy films that move ice crystals around. The result is a variety of ice crystals swirling across the surface of a soap bubble as it freezes, making it look somewhat similar to a snow globe.

Nanowire liquid pumps

This fascinating video is from a paper titled "Nanowire liquid pumps" published in Nature Nanotechonology. It demonstrated that the outer surface of a nanowire is able to transport liquid. When liquid flows on the surface, it can flow as the thin film flows or the discrete beads. The former flow can be described by the well-known thin film instability while the latter is due to Rayleigh-Plateau Instability. In the thin-film instability, a minimum thickness of the thin film is achieved due to the repulsive intermolecular forces, which prevents the breakup of the thin film. This paper also shows that there is a critical film thickness of ∼10 nm separating two flow mechanisms.

 

 

From RGD'31: Report on an investigation on periodically patterned radiometric pumps

I recently attended the 31st International Symposium on Rarefied Gas Dynamics held at the University of Strathclyde in Glasgow, Scotland.  The event was a great success in all its facets.  There was a remarkable turnout and both the oral presentations and poster display were of the highest scientific level.  During the week-long event, we were delighted with several invited talks by some of the world’s leading figures in the field of rarefied gas dynamics.  Several members of the Micro- and Nano-Flows Group for Engineering based at Edinburgh, Warwick, or Daresbury, participated in the symposium in a variety of roles.

 

In this blog entry, I want to focus on a talk I attended, entitled “Periodically patterned radiometric pumps – Novel configurations and further applications” by A. Lotfian, E. Roohi, and S. Stefanov. It was delivered by Dr. S. Stefanov of the Bulgarian Academy of Sciences.  The worked consisted on the numerical simulation of the flow and temperature fields of the gas in the channel formed between a patterned or ratchet surface on one side and a flat or, alternatively, ratchet surface on the other.  The radiometric pumping effect is generated by imposing a uniform temperature difference between both surfaces and by periodically changing the reflective property of the surface in the ratchet, namely, by alternating between a section reflecting specularly and another diffusely. Another configuration studied consisted of alternating a hot surface section with a cold one on the same side.  The effect of these perturbations is to create a net force and flow along the direction of the surfaces.  This net force can be used to generate motion if one of the surfaces is allowed to move, such as with a rotor.

 

In total, they investigated seven novel configurations.  A configuration very similar to the ones considered in this work can be seen here. For the analysis, they used the solver dsmcFoam+, an implementation of Direct Simulation Monte Carlo (DSMC) in the open source package OpenFoam+, parallelized using MPI.  They found that the maximum pumping velocity is observed in a zigzag channel with teeth machined on both sides.  On the other hand, the maximum radiometric force was attained in a channel with a flat wall on one side and double ratchets on the other wall.

Details about a numerical study like the one described in this entry can be found in this open-access article.

AIMS Conference (5-9 July 2018, Taipei, Taiwan)

On July 5/9, I took part to the 12th AIMS Conference on Dynamical Systems, Differential Equations and Applications in Taipei.
This conference aims at fostering and enhancing interactions among mathematicians and scientist in general. It has featured 135 special sessions with a broad range of topics. Keynote lectures were given by famous mathematicians (A. Buffa, V. Calvez, S. Peng, J. Ball, just to cite a few).

AIMS Conference schedule at a glance

I was in particular interested to two special sessions devoted to kinetic theory: "Models and Numerical Methods in Kinetic Theory" (where I was invited to give a talk) and "Kinetic and Related Equations: Collisions, Mean Field, Organized Motion".
Although kinetic equations have been traditionally applied to rarefied gas dynamics and plasma physics, these special sessions have confirmed an emerging trend in kinetic theory, namely the use of its theoretical framework to study topics in fields which are apparently far from fluid dynamics like the emergence of organized collective behaviour in vehicular traffic, crowds, swarms, social systems and biology. This wide range of new applications and the benefits that these studies can potentially bring to the society has significantly revived the interest in kinetic theory.
A detailed description of sessions along with the abstracts of presented talk can be found at the conference's website (http://aimsciences.org/conferences/2018/).

Overall, I was pleased to partecipate to the conference. The only negative aspect was that typhoon Maria stroke Taipei the day I had to flight back home. As a consequence, my flight was delayed and I was forced to spend almost two days segregated in hotel!

Musical Boiling

Boiling is an important feature in many engineering processes, such as in the steam cycle in many power plants. It is also a highly multiscale phenomenon, with boiling bubbles nucleating on nanoscale features of solid substrates and growing to sizes of the order of mm. Researchers at Massachusetts Institute of Technology (MIT) have demonstrated the drastic effects that alterations at the nanoscale can have on boiling nucleation with the use of surfactants. By applying a voltage to certain parts of a substrate, the surfactants effectively render the substrate hydrophobic and can rapidly induce boiling at very specific regions of the substrate.

The rate at which boiling can be switched on or off is also particularly impressive, although as we all know the ultimate test for how fast scientists can control a process is by syncing it up with classical music… MIT calls this piece “Ode to Bubbles”, enjoy!

Cho, H. J. et al. Turning bubbles on and off during boiling using charged surfactants. Nat. Commun. 6:8599 doi: 10.1038/ncomms9599 (2015)

OpenFOAM's I/O Problem (and solution)

Much of the MNF group's research output has been based around our solvers (mdFoam+ and dsmcFoam+) which are written in the OpenFOAM software framework. OpenFOAM is well known and well acknowedged as a very flexible and stable environment to develop new solvers, however it has a bit of a reputation for scaling badly on big super computers, leaving people to assume it should only be used when your problem can be tackled by a stand-alone workstation or using only a few nodes on your favourite big HPC system. This blog post will talk about the new collated file format introduced into OpenFOAM 5.0 and how it might be the beginning of the end for this mentatility.

The question is, where has this perception come from and, more importantly, is it right? If you search for the issue of OpenFOAM scalability on HPC then you will find numerous articles and topics, what is interesting though is how few are a) looking at massive scalability (most consider running on a few CPUs) b) how few recent articles there. 

The question therefore is whether OpenFOAM actually does perform badly on HPC system or is it an out of date perception. This is a hard one to answer fully as OpenFOAM has been around for a good few decades and has a number of different solvers to consider. In theory, each should parallelise as well as the others as they are all built on top of same basic libraries, however of course some algorithms work better in parallel than others and some of the solvers may not have receieved the same attention as others. Generally speaking though the methods used in OpenFOAM are sound, it employs typical static domain-decomposed non-blocking MPI in most of its solvers and allows well-known decomposition libraries such as Scotch to be used to minimise communication overhead. Undoubtedly this could all be optimised better if it were to receieve lots of attention from the HPC community but are there any other problems blocking this?

The MNF group runs many of its simulations on the UK's national HPC service Archer, run by the EPCC, a Cray XC30 machine. At the moment they provide access to OpenFOAM 4 on their system. Arguably OpenFOAM has a bad reputation for use on this system but the same problems are repeated on many systems, especially those that use a Lustre parallel file system and that is the way that OpenFOAM creates and deals with its files.

For every MPI process created, a new folder is also created and a set of files. In cases where lots of output is created during a simulation this can easily mean there are thousands of files per processor created on disk, Archer provides a hard limit per user on the number of files that can be created in their storage and also that they can have open in memory at any one time, parallel runs using OpenFOAM quickly exceed this and can have a major impact on the parallel file system for other users if the limits wern't there, as a result of this OpenFOAM has developed a bad reputation. It is worth noting that this approach is an entirely valid, if outdated, way of dealing with I/O when using MPI.

The good news is that, as of OpenFOAM 5.0, this has been changed and now there is a new way of writing files to disk known as the collated file format. This is a simple idea, rather than each MPI process creating its own folder, there is now just one set of files written by the master process and all other processes transfer data back via MPI. If you get hold of the latest development version via the OpenFOAM-dev repository then this has been further developed so you can mark individual MPI processes as "master node" writers to spread the load and reduce communication overhead as then processes only need to talk to each other within the same node. Therefore, if you were running on 48 nodes of Archer then you would have 1152 MPI processes with 24 on each node, so you would have 48 sets of files instead of 1152. This is really quite significant as if you assume there are 1000 files per set by the end of a simulation then you have 48,000 rather than 1,152,000!

We have done some basic testing and have found using the new file format to be about 50% faster on Archer using the flow past a motorbike tutorial case with simpleFoam and 48 nodes. 

Of course the really exciting thing about this development is that the HPC community can now really get stuck in to the challenge of properly benchmarking OpenFOAM over many more MPI ranks than it has previousely attempted as cases now scale, this will therefore hopefully lead to rapid development of the underlying MPI approach and only serve to increase performance of OpenFOAM across all of its solvers, including the MNF group codes!

Thermal fluctuations in nanoscale fluid interfaces

My current research focuses on the hydrodynamics fluctuations in nano-jets. The earliest research (Moseler M., Sci. 2000) found new double-cone rupture profiles due to thermal fluctuations (molecule motions), which the Navier-Stokes models failed to predict. Our research shows that these fluctuations not only affect the final rupture profiles but also change the wavelengths of perturbations. 

Moreover, I have found that thermal fluctuation effect widely exists in the nanofluids, especially those with the interfaces. So I summarized some previous research in the figure above and listed the literature (links) below.

(1)Nanojet flows:  
[1.1] Moseler M.,  2000
[1.2] Egger J., 2002
[1.3] Hennequin Y., 2006
[1.4] Kang W., 2007
[1.5] Petit J., 2012
(2)Drop coalescence
[2.1] Dirk G. A., 2004
(3)Fluid mixture
[3.1] Kadau k., 2007
(4) Moving contact lines
[4.1] Perrin H., 2016
[4.2] Belardinelli D., 2016
[4.3] Davidovitch B., 2005
(5) Bubble
[5.1]  Gallo M., 2018
(6) Thin film
[6.1] Grun G., 2005
[6.2] Fetzer R., 2007
[6.3] Diez J. A., 2016

Although the phenomena above is distinct, mathematical models were derived from the same equations, Landau and Lifshitz Navier-Stokes equations (LLNS). What's more, particle methods (MD or DSMC) can be employed to support the new physical models as numerical experiments.
Therefore, there are lots of opportunities for us to employ both math models and simulations to study this interesting topic. 

SWEP Workshop, Brighton

On May 17/18 Rohit and I gave invited talks at the inaugural Surface Wettability Effects on Phase Change Phenomena (SWEP) workshop in Brighton.  This was organised by Joel De Coninck, our first Visiting Scientist of the Programme, and Marco Marengo who are both experts in this field - their hope is that this workshop will become a yearly fixture.  They opened the workshop by reminding the audience of the incredible effects that wettability can have: adding just one layer of molecules to the top of a surface can completely change the shape of mm-sized drops that sit on top of them, which is the equivalent in scale of ants being able to change the shape of mountains (apologies for the poor quality photo)!

Rohit and I gave the last and first talks, respectively, with Rohit impressing the audience with his work on acoustofluidics whilst I spoke about 3 canonical problems involving kinetic effects in interfacial flows, including work with Mykyta (drop impact), Anirudh (drop evaporation) and Duncan.

There were many interesting presentations on a wide range of phase change phenomena.  I particularly enjoyed Carlo Antonini's talk "License to Freeze" which reviewed methods for controlling ice formation on surfaces (including an inverse Leidenfrost effect, where evaporation occurs from the underlying substrate rather than the impacting drop drop, which we could potentially simulate) and Daniel Attinger's talk on "What is the Optimum Wettability of a Pool Boiling Heater?", which carefully explained the experimental and theoretical challenges of understanding the subtle interplay between wettability, phase change and heat transfer driven by bubble formation at a (complex) solid surface.

All in all the workshop was very enjoyable and the level of scientific discussion was high (i.e. Rohit and I got grilled!) - I would recommend it to our group members in future years.

Polymer Flooding for Enhanced Oil Recovery

Only upto 40% of the original oil in place could be extracted from the oil reservoir using water flooding, the so-called primary recovery method . A large portion of oil is left behind as immobile ganglia. The enhanced oil recovery (EOR) techniques, the secondary recovery methods, are hence needed to extract the remaining oil. Polymer flooding is one such technique. In this EOR techniques, the injected water is supplemented with the long chain polymers which makes the injected water an visco-elastic fluid with a tunable viscosity. The primary purpose of adding polymer is to increase the viscosity of the flood water so that he mobility of injected water becomes less then that of oil which maximize the sweep efficiency, creating a smooth flood font without viscous fingering. This EOR technique has been successfully used to effectively recover the remaining oil from the reservoir, up to 30% of the original oil in place. 

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)