Impact of surface roughness on diffusion of confined fluids

TL;DR

This study uses molecular dynamics simulations to analyze how different surface roughness characteristics of confining boundaries affect the self diffusivity of confined hard-sphere fluids, providing predictive insights.

Contribution

It introduces a linear relationship between diffusivity ratios and wall separation for rough boundaries, enhancing understanding of confinement effects.

Findings

Diffusivity ratio depends linearly on reciprocal wall separation.

Roughness nature influences the slope of the diffusivity relationship.

Practical methods are proposed to predict fluid behavior in rough confinements.

Abstract

Using event-driven molecular dynamics simulations, we quantify how the self diffusivity of confined hard-sphere fluids depends on the nature of the confining boundaries. We explore systems with featureless confining boundaries that treat particle-boundary collisions in different ways and also various types of physically (i.e., geometrically) rough boundaries. We show that, for moderately dense fluids, the ratio of the self diffusivity of a rough wall system to that of an appropriate smooth-wall reference system is a linear function of the reciprocal wall separation, with the slope depending on the nature of the roughness. We also discuss some simple practical ways to use this information to predict confined hard-sphere fluid behavior in different rough-wall systems.