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Molecular Dynamics and Microfluidic Systems

“The present dissertation is a remarkable step towards the understanding of transport properties of concentrated polymer solutions on a microscopic level… really interesting and a joy to read… I find this an excellent dissertation (would rank a thesis like this in the top 10% in Europe), where new phenomena have been investigated which are at the moment at the forefront of this type of research.” J.K.G. Dhont, Head of Institute for Soft Matter, IFF ( Institute of Solid State Research ), Jülich, Germany.

“This thesis presents a very substantial amount of original work on the behaviour of complex fluids in thin channels. It is well written and presents interesting and valuable new results in a very important area of research. The results are carefully analysed and presented. I am sure that the results will lead to advances in the field.” D. Bernhardt, Senior Lecturer in Physical and Theoretical Chemistry, and Deputy Director of Nanoscale Science and Technology Centre, Griffith University, Brisbane, Australia.

“This is a beautiful thesis” and “All-in-all the most complete thesis I’ve ever seen.” I.K. Snook.

Spicks & Specks, 2nd March 2006

Inspired by the vaunted prospect of a major growth in micro-fluidic techniques and technologies, this study examines the physical responses of various fluids to simple micro- and nano-scale confinement. Microscopic molecular dynamics (MD) simulations are used to probe channel-widths and observe properties that are inaccessible to contemporary experiments and analyse behaviours that are unobservable through macroscopic computational fluid dynamics.

This work advances upon previous MD studies of single -omponent fluids between very-closely-spaced walls, by simulating fluids confined to geometries that are both wide enough to conform to classical predictions and narrow enough to see these predictions fail. Additionally, this analysis adds a second component to the fluid which, in each system simulated, differs substantially in size, mass, or structure from the first.

Through equilibrium simulations of the fluids when unconfined, this study evaluates important thermodynamic, solution and transport properties such as the concentration gradient of the chemical potential, the degree of ideality and the values of mass/heat diffusion coefficients. These properties, especially the latter, are then applied to the analysis of the fluid when experiencing inhomogeneous Poiseuille flow between solid (but explicitly molecularly modelled) parallel walls. Thus Navier-Stokes-based predictions of the velocity, concentration and temperature profiles within the fluid are generated, without recourse to free fitting parameters. This analysis relies on a unique application of equilibrium-calculated properties to the generation of non-equilibrium flow predictions.

These classical predictions, when compared to the results of the Poiseuille flow simulations are successful in describing flows that are comparatively slow, in systems that are comparatively wide. Classical predictions are shown to begin to fail when the width of the channel is reduced. Where the simulated working fluid contains large colloid particles, reducing the separation between the confining walls sees molecular effects becoming increasingly influential, and conspicuous features such as layering and wall-slip become discernible. Furthermore, as the flow rate is increased, even in the wider systems, assumptions behind the classical analysis begin to break down. This is illustrated in the simulations where the model fluid contains short chain polymer molecules in solution. Here,, greater external forces are required to induce the fluid to flow, so that decreasing the polymer content leads to a strong increase in flow rate that carries the system out of the classical regime. It is shown that increasing the flow rate can lead to a strong variation in temperature across the profile, with consequent non-classical spatial variations in the density and viscosity of the fluid.

Some of the simulated polymer solutions also show a profound variation in concentration across the profile (reaching an extreme in the dilutest solution, with complete molecular depletion over significant regions near the walls) which is also taken into account in an evaluation of the local viscosity across the fluid.

It is shown that subsuming all of these variables into a more sophisticated prediction of the velocity profile gives an improved description of the fluid behaviour at these higher flow rates.

Unlike the spherical colloid, the polymer chains have additional significant orientational and conformational aspects to their behaviour. When confined to a very narrow geometry, the polymer solutions are especially prone to molecular rotation, although this effect is shown to be suppressed when the chains are extended. The molecular conformations and interactions in this narrow system also lead the polymer solutions to exhibit slip at the walls, with profiles tending towards plug flow.

The fluids modelled here are molecularly very different from each other, and are simulated over a wide range of confinements. Comparison between them is therefore assisted through the evaluation of dimensionless numbers describing some of their various behaviours. Through these dimensionless numbers, the results of this work are also compared with relevant micro-fluidic experiments. In the cases of both the spherical colloid and the polymer solutions, overall flow behaviour in the widest channels is shown to reliably replicate appropriate experimental results, while providing detailed information on aspects of the flow which are experimentally elusive.

Members of RMIT’s comsim research group know that there is more than one way to study fluid flow.
Check out Akin Budi‘s website for more info on the great flood of 2003.

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