Tracer diffusion of hard-sphere binary mixtures under nano-confinement

TL;DR

This paper investigates how nano-confinement affects diffusion in binary fluid mixtures, revealing new mechanisms for transverse and effective diffusion due to boundary roughness and channel geometry.

Contribution

It derives a friction tensor expression for confined mixtures and introduces a one-dimensional diffusion model incorporating geometrical entropic effects.

Findings

Boundary roughness induces transverse diffusion mechanisms.

Effective diffusion can surpass planar channel diffusion due to confinement.

A reduced 1D diffusion equation with entropic terms models the system.

Abstract

The physics of diffusion phenomena in nano and micro channels has attracted a lot of attention in recent years, due to its close connection with many technological, medical and industrial applications. In the present paper we employ a kinetic approach to investigate how the confinement in nanostructured geometries affects the diffusive properties of fluid mixtures and leads to the appearance of properties different from those of bulk systems. In particular, we derive an expression for the friction tensor in the case of a bulk fluid mixture confined to a narrow slit having undulated walls. The boundary roughness leads to a new mechanism for transverse diffusion, and can even lead to an effective diffusion along the channel larger than the one corresponding to a planar channel of equivalent section. Finally we discuss a reduction of the previous equation to a one dimensional effective diffusion equation in which an entropic term encapsulates the geometrical information on the channel shape.