Micro & Nano Flows for Engineering

The micro & nano flows group is a research partnership between the Universities of Warwick and Edinburgh, and Daresbury Laboratory. We investigate gas and liquid flows at the micro and nano scale (where conventional analysis and classical fluid dynamics cannot be applied) using a range of simulation techniques: molecular dynamics,  extended hydrodynamics, stochastic modelling, and hybrid multiscaling. Our aim is to predict and understand these flows by developing methods that combine modelling accuracy with computational efficiency.

Targeted applications all depend on the behaviour of interfaces that divide phases, and include: radical cancer treatments that exploit nano-bubble cavitation; the cooling of high-power electronics through evaporative nano-menisci; nanowire membranes for separating oil and water, e.g. for oil spills; and smart nano-structured surfaces for drag reduction and anti-fouling, with applications to low-emissions aerospace, automotive and marine transport.

 

EPSRC Programme Grant in Nano-Engineered Flow Technologies

Our work is supported by a number of funding sources (see below), including a 5-year EPSRC Programme Grant (2016-2020). This Programme aims to underpin future UK innovation in nano-structured and smart interfaces by delivering a simulation-for-design capability for nano-engineered flow technologies, as well as a better scientific understanding of the critical interfacial fluid dynamics.

We will produce software that a) resolves interfaces down to the molecular scale, and b) spans the scales relevant to the engineering application. As accurate molecular/particle methods are computationally unfeasible at engineering scales, and efficient but conventional fluids models do not capture the important molecular physics, this is a formidable multiscale problem in both time and space. The software we develop will have embedded intelligence that decides dynamically on the correct simulation tools needed at each interface location, for every phase combination, and matches these tools to appropriate computational platforms for maximum efficiency.

This work is strongly supported by nine external partners (see below).

Current Funding

  • “Nano-Engineered Flow Technologies: Simulation for Design across Scale and Phase” EPSRC Programme Grant EP/N016602/1 01/16-12/20 (£3.4M)
  • “The First Open-Source Software for Non-Continuum Flows in Engineering” EPSRC grants: EP/K038427/1 K038621/1 K038664/1 07/13-06/17 (£0.9M)
  • “Multiscale Simulation of Interfacial Dynamics for Breakthrough Nano/Micro-Flow Engineering Applications” ARCHER Leadership Project 11/15-10/17 (£60k in supercomputer computational resource)
  • “Skating on Thin Nanofilms: How Liquid Drops Impact Solids” Leverhulme Research Project Grant 08/16-08/19 (£146k funding a 3-year PDRA)

Partnerships

  • Airbus Group Ltd
  • AkzoNobel
  • Bell Labs
  • European Space Agency
  • Jaguar Land Rover
  • Oxford Biomedical Engineering (BUBBL)
  • TotalSim Ltd
  • Waters Corporation

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Latest news and blogs

Dr Benzi John, Senior Computational Scientist, Daresbury Laboratory

Benzi John (PI) and David Emerson (Co-I) have successfully won about 32, 000 KAU (~ £18,000) frunding from EPSRC to run large-scale simulations on ARCHER. The project titled "High fidelity non-equilibrium DSMC flow simulations at scale using SPARTA" will run for 12 months, starting from August 1st 2017. More information here.

Dr. Duncan Lockerby

Prof. Duncan Lockerby, University of Warwick

We're sad to say goodbye to Kayleigh Hyslop, who has been our Programme Administrator for the past year or so. Kayleigh is off to teach scuba diving in the British Virgin Islands (wouldn't we all, if we could?). We wish her all the best and thank her for her hard work in setting up the Programme; and particularly for making our `research creativity' day such a success.

The good news is that Andrea Cooper joined us at the end of May, and is settling into her new role fast. Andrea will be working two days a week, based in Engineering at Warwick. She's looking forward to working with everyone, and I'm sure you'll hear more from her soon!

Dr Juan C. Padrino, Research Fellow, University of Warwick

MNF will be present in the upcoming 3rd UK InterPore Conference on Porous Media​ to be held at the University of Warwick, Coventry, UK, on 4 and 5 September 2017.  On the afternoon session of 5 September, Juan C. Padrino will give a talk on multiscale modeling of diffusive transport in complex networks.

Sree Hari P D , PhD Student, University of Edinburgh

Initial configuration of an atomic system is crucial in MD simulations. Further away the initial configuration from its equilibrium position, longer will it take to reach the equilibrium state. Longer simulation means burning up more and more computational resources.

For example, If you want to simulate a drop spreading on a Platinum surface using MD and you are not interested in the transient dynamics, one obvious way is to begin with a wall and a cube of water molecules placed just above it. The attraction from the wall will bring down the cube and the “droplet” will spread.

But, if you already had a hemispherical arrangement for water molecules instead of a cube, the system would be nearer to its equilibrium state at t=0. This will only take much less time to attain equilibrium and thus will help save a lot of computational resources.

The working procedure is very simple as well. You can download and install Packmol from http://www.ime.unicamp.br/~martinez/packmol/home.shtml. Suppose we want 2000 water molecules randomly arranged inside a sphere of radius 25 Angstroms, all we need is two files:

File 1: tip4p.xyz

This file contains the atomic coordinates of two Hydrogen and an Oxygen atom according to TIP4P configuration of water molecule. The first line represents the total number of atoms to follow. Second line is neglected.

#######################

3

 

O         0.0       0.0       0.0

H          0.9572 0.0       0.0

H          -0.239  0.9266 0.0

#######################

File 2: input.inp

The packmole executable reads this file as an instruction to make initial configurations. In the file, tolerance is the average distance between two molecules and filetype is the output file format. The first three arguments of inside sphere denote the coordinates of centre of the sphere and last argument is the radius. You can also arrange water molecules inside a box, cylinder etc.

#######################

tolerance 2.0

filetype xyz

output waterSphere.xyz

structure tip4p.xyz

            number 2000

            inside sphere 0.0 0.0 0.0 25

end structure

#######################

Now, open a terminal in the packmol folder and execute “./packmol <input.inp”.

This will create a file named waterSphere.xyz, which will have 2000 water molecules arranged inside a sphere centred at (0,0,0) with radius 25 Angstroms. Next step is to write a C++/MATLAB script to read this output file and print it in a format that can be read by the MD software that you use. If you find packmol useful, don't forget to site the original work.

Dr Mykyta Chubynsky, Research Fellow, University of Warwick

In the famous Leidenfrost effect, liquid drops levitate over a heated solid surface without touching it. For this effect to occur, the surface should be heated well above the boiling temperature of the liquid. On the other hand, levitation over a liquid surface can occur for much lower surface temperatures. We have all seen this when drinking hot tea or coffee. There is often what looks like a misty film over the surface. It is formed when the evaporating water condenses and some of the resulting microdrops levitate over the surface. These drops are remarkably uniform in size and form arrays with a high degree of order. Leidenfrost drops do not form such ordered arrays. In a recent article in Physical Review Letters, a group of Russian physicists was able to show that ordered drop arrays can occur over solid surfaces as well, well below the Leidenfrost temperature, and give an explanation for this phenomenon. To create such arrays, they started with a layer of water on top of a copper substrate.  With a short pulse of an air jet they created a millimetre-sized dry spot on the surface. The spot stayed dry after the air flow ceased, because the surface was rough enough that the contact line was pinned. Then they heated the substrate to 85 C. As expected, they observed an array of microdrops above the liquid, but some of the hovering drops moved over the dry spot and formed an ordered array there as well, though with less order.

In order to explain the phenomenon, the authors considered the dynamics of evaporation of the drops and associated air flows. When a drop is evaporating in air, the vapour concentration is, obviously, higher near the drop. However, the total concentration of molecules (air plus vapour) is roughly the same everywhere, and therefore the air molecule concentration is lower near the drop surface and there should be diffusion of air molecules towards the surface. But since the air molecules cannot penetrate the drop, there should be a counterbalancing convective flow away from the drop surface, which is called Stefan flow, after the Austrian-Slovene scientist of the Stefan-Boltzmann law fame. The authors claim that it is this flow that repels the drops from the liquid or solid surface and from each other, making them levitate and form ordered arrays.

While the authors do some calculations to justify their explanation, there are approximations involved, so simulations avoiding these approximations may be useful. Moreover, the sizes of the drops they observe, the distances between them and the heights above the substrate are about 5-10 microns, just large enough for continuum hydrodynamics to apply. It is plausible that smaller drops may be observed (this depends, in particular, on the temperature to which the substrate is heated), in which case taking gas-kinetic effects into account may be warranted.

This work has also been covered in Physics World.

Livio Gibelli, Research Fellow, University of Warwick

A chain fountain is the name given to the counterintuitive phenomenon where a long bead chain
appears to defy gravity by first leaping out of its container before falling to the ground.

This became known as the Mould effect, after a British science presenter, Steve Mould, who made the experiment famous with a video that went viral on YouTube.
Apparently, he discovered the chain fountain phenomenon while looking for a way to explain at the molecular level the capability of viscoelastic fluid to pour itself, the so-called open siphon effect.

In this funny TED talk, Steve Mould recounts his discovery and investigation into this entertaining and counterintuitive phenomenon.


The physics behind the chain fountain is non-trivial.

At first sight one could be tempted to explain the phenomenon by observing that the falling chain has downward momentum, causing an upward momentum in beads leaving the container. This, in turn, makes them leap before gravity can slowly reverse their momentum. However, if inertia causes the flowing fountain, the chain would be stationary at the top of the curve, while this is not the case.

It has been later proposed that the fountain is not driven by inertia, momentum, or gravity but rather by an anomalous push force exerted by the container on the link of the chain about to come into motion.
(http://rspa.royalsocietypublishing.org/content/470/2163/20130689).
The authors of the paper also posted a video on the Royal Society Youtube channel where the educational value of the demonstration and the analysis is highlighted.

Since it was first brought to widespread attention by Mould, many explanations of the chain fountain have been put forward and several papers have been published, even recently (http://aapt.scitation.org/doi/abs/10.1119/1.4980071, https://journals.aps.org/pre/abstract/10.1103/PhysRevE.89.053201, https://arxiv.org/abs/1612.09319).
However, a complete description of the mechanisms at play appears to be still lacking.