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. 

The Weissenberg Effect

Take a look at this video on the Weissenberg effect.

When stress is applied to a fluid, a certain amount of strain, or deformation, is observed. In particular, for a Newtonian fluid, viscous stresses that arise from the flow are linearly proportional to the local rate of deformation over time. It is determined from this that for Newtonian fluids (such as water and oil), the viscosity of the fluid does not depend on the applied stress. So when a rotational stress is applied (as in the video) at the bottom of a Newtonian fluid, below an air-liquid interface, the resulting flow is dominated by inertial and gravity-effects which causes the fluid to flow down toward the source of the stress and radially outward. For pseudoplastics (such as grease), a type of Non-Newtonian fluid where the viscosity decreases as the applied stress increases, applying the same rotational stress will decrease viscosity near the source, which causes inertia and gravity to no longer dominate the flow, causing the fluid to flow upward and bulge at the air interface.

A fun instance of this effect is known as 'rod-climbing'. Rotating a rod inserted into a pseudoplastic causes the polymer chains that make up the fluid to congregate around the regions of highest stress and orient themselves in the direction of shearing, meaning that chains closer to the rod (where stress is highest) are stretched less than those chains further away, and so occupy 'lower states of energy'. The chains' desire to reach these regions of low energy creates an inward normal force and so the fluid goes in the only direction is can - UP THE ROD.

British Applied Mathematics Colloquium 2018, St Andrews

This week I attended the BAMC in St Andrews. I have given a talk as part of the “Multiscale  Analysis of Porous Media” mini-symposium. I spoke about modelling and upscaling multiphase flow in porous media and the session also included talks on upscaling the poro-elastic properties of soil, the growth of potatoes and using image-based modelling to investigate plant-fertiliser interaction.

There are a wide range of topics were presented and discussed at BAMC, which has combined with the UK Magnetohydrodynamics Meeting this year. In my own work I am modelling phenomena at the millimetre and micrometre scale, but I have had the opportunity to attend interesting talks that aim to model much larger scales, such as the formation of the Milky Way and waves in oceans. Mathematical biology was also a popular topic. There were presentations and posters on the more familiar topics of drop impacts and optimisation.

I have attended talks on the history of mathematics and have learnt that 19th Century mathematicians could be really mean to each other! Even going as far as writing whole books to explain just how wrong they thought the other person was. Luckily, most of the attendees here did not share this attitude and were friendly.

Collapsing Bubbles and Microjets

A spherical bubble becomes unstable when it is subject to forces such as gravity, or when it is in proximity of surfaces such as walls or free-surfaces. The presence of such surfaces alters the surrounding pressure field, causing the bubble to lose its original sphericity, fold on itself and collapse. As the bubble collapses, some interesting and noteworthy flow phenomena take place. The detailed high-speed visualisations in the following video reveal some of these complex dynamics for a bubble collapsing close to a free surface. 

In this case, it is the proximity of the free surface that causes the violent collapse. Once the laser-generated bubble grows to its maximum size, the collapse stage starts as the bubble begins to lose its sphericity. The upper part of the bubble accelerates downwards and becomes pierced by a liquid microjet. The jet itself tends to be difficult to observe visually, but its location can be identified by the protrusion it leaves when pushing part of the bubbles gaseous contents downwards.

When the jet impacts on the opposite bubble wall, a set of shock waves are emitted. In this case, the jet is so wide that it touches the interface not at a single point but in a ring, thus making the shock wave emission mechanism even more complex. Following jet impact, the bubble is separated into two parts, as the vapour cavity swept along by the jet becomes detached from the main toroidal bubble. Upon collapse of the main bubble, a stronger second set of shock waves is emitted which is reflected by the free surface causing excitation of nearby small bubbles.

More details here: https://doi.org/10.1063/1.4931098

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 interesting facts about water is that it does not always freeze at 273.15 K. Actually, 'pure water', which is water with no impurities and free of nucleation sites, can stay in liquid form for temperatures as low as 224.8 K.

When pure water is supercooled it is very easy to freeze it, because any iteraction with the water molecules in that state can result in the formation of nucleation sites, that will instantly result in the freezing of 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).

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)