Hard sphere fluids in annular wedges: density distributions and depletion potentials

TL;DR

This paper investigates the behavior of hard sphere fluids confined in annular wedges, revealing how density distributions and depletion forces depend on size ratios and packing fractions, with implications for colloidal interactions.

Contribution

It introduces a detailed analysis combining density functional theory, integral equations, and simulations to understand depletion interactions in confined hard sphere fluids.

Findings

Density distribution resembles a 2D disk system near an obstacle.

Depletion force has a geometric interpretation for large size ratios.

Deep attractive depletion potential observed at high solvent packing fractions.

Abstract

We analyze the density distribution and the adsorption of solvent hard spheres in an annular slit formed by two large solute spheres or a large solute and a wall at close distances by means of fundamental measure density functional theory, anisotropic integral equations and simulations. We find that the main features of the density distribution in the slit are described by an effective, two--dimensional system of disks in the vicinity of a central obstacle. For large solute--solvent size ratios, the resulting depletion force has a straightforward geometrical interpretation which gives a precise "colloidal" limit for the depletion interaction. For intermediate size ratios 5...10 and high solvent packing fractions larger than 0.4, the explicit density functional results show a deep attractive well for the depletion potential at solute contact, possibly indicating demixing in a binary mixture at low solute and high solvent packing fraction.