Relating thermodynamic quantities of convex-hard-body fluids to the body's shape

TL;DR

This paper develops a formalism to relate thermodynamic properties of convex hard-body fluids to their shape, using a perturbative approach based on mapping to spherical particles, and validates it with simulations.

Contribution

It introduces a systematic perturbative method to calculate thermodynamic quantities of convex hard-body fluids based on shape anisotropy.

Findings

Equation of state derived as a functional of shape

Phase transition shifts linearly with shape anisotropy

Validation through Monte Carlo simulations

Abstract

For a fluid of convex hard particles, characterized by a length scale $\sigma_\text{min}$ and an anisotropy parameter $\epsilon$, we develop a formalism allowing one to relate thermodynamic quantities to the body's shape. In a first step its thermodynamics is reduced to that of spherical particles. The latter have a hard core of diameter $\sigma_\text{min }$ and a soft shell with a thickness $\epsilon \sigma_\text{min}/2$. Besides their hard core repulsion at $\sigma_\text{min }$ they interact by effective entropic forces which will be calculated. Based on this mapping, a second step provides a perturbative method for the systematic calculation of thermodynamic quantities with the shape anisotropy $\epsilon$ as smallness parameter. In leading order in $\epsilon $, the equation of state is derived as a functional of the particle's shape. To illustrate these findings, they are applied to a one- and two-dimensional fluid of ellipses and compared with results from different analytical approaches, and our computer simulations. The mapping to spherical particles also implies that any phase transition of spherical particles, e.g., the liquid-hexatic transition, also exists for the nonspherical ones, and shifts linearly with $\epsilon $ for weak shape anisotropy. This is supported by our Monte-Carlo simulation.