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

TL;DR

This study shows that a generalized available volume measure, predicted by density functional theory, can reliably forecast the self diffusivity of confined fluids across various geometries and conditions, aiding in understanding their dynamics.

Contribution

It introduces a new static measure of available volume that correlates strongly with confined fluid dynamics, enabling semi-quantitative predictions of diffusion coefficients.

Findings

Available volume measure correlates with self diffusivity in confined fluids.

Density functional theory can predict static properties used to estimate dynamics.

The correlation holds across different geometries and confinement levels.

Abstract

Although density functional theory provides reliable predictions for the static properties of simple 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, 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 study this issue 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 $10^3$ equilibrium state points. We examine three different confining geometries (slit pore, square channel, and cylindrical pore) and the effects of packing fraction and particle-boundary interactions. Although density fails to predict some key qualitative trends for the dynamics of confined fluids, we find that a new generalized measure of available volume for inhomogeneous fluids strongly correlates 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 this static property can be used together with knowledge of bulk fluid behavior to estimate the diffusion coefficient of confined fluids under equilibrium conditions.