Tuning Density Profiles and Mobility of Inhomogeneous Fluids

Abstract

Density profiles are the most common measure of inhomogeneous structure in confined fluids, but their connection to transport coefficients is poorly understood. We explore via simulation how tuning particle-wall interactions to flatten or enhance the particle layering of a model confined fluid impacts its self-diffusivity, viscosity, and entropy. Interestingly, interactions that eliminate particle layering significantly reduce confined fluid mobility, whereas those that enhance layering can have the opposite effect. Excess entropy helps to understand and predict these trends.Fluids confined to narrow spaces adopt an inhomogeneous distribution of density due to the interactions between the fluid particles and the boundaries. This density profile provides a basic means for characterizing confined fluid structure, and in particular how it differs from that of a homogeneous bulk system. For simple confined fluids, the density profile can be quantitatively predicted using classical density functional theory. However, differences between confined and bulk fluids are not limited to static structure. The former also flow, diffuse, and conduct heat at different rates than the latter. Unfortunately, a theory that can reliably predict transport coefficients has yet to emerge. In fact, even an intuitive understanding of how the density profile of a confined fluid connects to its dynamics is lacking.For bulk fluids, semi-empirical structure-property relations have helped to correlate and predict transport coefficients (see, e.g., [1,2,3]). Specifically, changes in thermodynamic state variables that increase short-range structural order of fluids are also known to decrease their mobility in a simple, quantifiable way. This is true even for systems that exhibit anomalous dynamical behavior, such as cold liquid water (where viscosity decreases upon compression) [4,5,6] or concentrated colloidal suspensions (where interparticle attractions increase mobility) [5,7]. Naïve extrapolation of this idea might lead one to suspect that inhomogeneous fluids with highly structured (e.g., layered) density profiles would tend to be more viscous and less diffusive than more spatially uniform fluids. Is that indeed the case? Here, we explore this issue quantitatively. Specifically, we use molecular simulation to investigate the relationship between the transport coefficients of an inhomogeneous fluid and its density profile, the latter of which can be modified in a precise way through the interactions of the fluid particles with the confining boundaries.A key empirical observation motivating this study is the existence of an isothermal correlation between the self-diffusion coefficient of simple inhomogeneous fluids and excess entropy (relative to ideal gas), which is approximately obeyed across a wide range of confining environments [8,9,10]. Since the magnitude of the excess entropy is itself a measure of structural order [11] , the aforementioned correlation is effectively a structure-property relationship. But how does excess entropy conn...

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