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Available states and available space: Static properties that predict dynamics of confined fluids

Published

Author(s)

Vincent K. Shen, Gaurav Goel, William P. Krekelberg, Mark J. Pond, Jeetain Mittal, Jeffrey R. Errington, Thomas M. Truskett

Abstract

Although classical density functional theory provides reliable predictions for the static properties of simple equilibrium fluids under confinement, a theory of comparative accuracy for the transport coefficients has yet to emerge. Nonetheless, there is evidence that knowledge of how confinement modifies static behavior can aid in forecasting dynamics. Specifically, recent molecular simulation studies have shown that the relationship between excess entropy and self-diffusivity of a bulk equilibrium fluid changes only modestly when the fluid is isothermally confined, indicating that knowledge of the former might allow semi-quantitative predictions of the latter. Do other static measures, such as those that characterize free or available volume, also strongly correlate with single-particle dynamics of confined fluids? Here, we investigate this question for both the single-component hard-sphere fluid and hard- sphere mixtures. Specifically, we use molecular simulations and fundamental measure theory to study these systems at approximately 1000 equilibrium state points. We examine three different confining geometries (slit pore, square channel, and cylindrical pore) and the effects of particle packing fraction and particle-boundary interactions. Although average density fails to predict some key qualitative trends for the dynamics of confined fluids, we find that excess entropy as well as a new generalized measure of available volume for inhomogeneous fluids strongly correlate with the self-diffusivity across a wide parameter space in these systems, approximately independent of the degree of confinement. An important consequence, which we demonstrate here, is that density functional theory predictions of these static properties can be used together with knowledge of bulk fluid behavior to semi-quantitatively predict the diffusion coefficient of confined fluids under equilibrium conditions
Citation
Journal of Statistical Mechanics: Theory and Experiment
Volume
P04006

Keywords

statistical mechanics, hard spheres, confined fluids, thermodynamics, dynamics
Created April 8, 2009, Updated February 19, 2017