Methods to Compute Pressure and Wall Tension in Fluids containing Hard Particles

TL;DR

This paper evaluates different computational methods for accurately determining pressure and wall tension in colloidal fluids with hard particles, addressing challenges posed by anisotropic pressure tensors in confined systems.

Contribution

It provides a comparative analysis of multiple methods to compute pressure and wall tension, highlighting their limitations and achieving good agreement among them.

Findings

Methods can accurately compute pressure tensor components

Wall tension can be reliably extracted despite anisotropy

Good mutual agreement among different computational approaches

Abstract

Colloidal systems are often modelled as fluids of hard particles (possibly with an additional soft attraction, e.g. caused by polymers also contained in the suspension). in simulations of such systems, the virial theorem cannot be straightforwardly applied to obtain the components of the pressure tensor. In systems confined by walls, it is hence also not straightforward to extract the excess energy due to the wall (the "wall tension") from the pressure tensor anisotropy. A comparative evaluation of several methods to circumvent this problem is presented, using as examples fluids of hard spheres and the Asakura-Oosawa model of colloid-polymer mixtures with a size ratio $q=0.15$ (for which the effect of the polymers can be integrated out to yield an effective attractive potential between the colloids). Factors limiting the accuracy of the various methods are carefully discussed, and controlling these factors very good mutual agreement between the various methods is found.