Isotropic-nematic transition in hard-rod fluids: relation between continuous and restricted-orientation models

TL;DR

This paper investigates how discretizing orientations in hard-rod fluid models affects phase behavior, revealing artificial phases that depend on the number of allowed orientations and their sampling accuracy.

Contribution

It demonstrates the emergence of artificial nematic phases in discretized models and analyzes their dependence on the number of orientations and mixture composition.

Findings

Artificial nematic phase appears with coarse orientation sampling.

Increasing orientations reduces artificial phase influence.

Discretization impacts phase coexistence and transition accuracy.

Abstract

We explore models of hard-rod fluids with a finite number of allowed orientations, and construct their bulk phase diagrams within Onsager's second virial theory. For a one-component fluid, we show that the discretization of the orientations leads to the existence of an artificial (almost) perfectly aligned nematic phase, which coexists with the (physical) nematic phase if the number of orientations is sufficiently large, or with the isotropic phase if the number of orientations is small. Its appearance correlates with the accuracy of sampling the nematic orientation distribution within its typical opening angle. For a binary mixture this artificial phase also exists, and a much larger number of orientations is required to shift it to such high densities that it does not interfere with the physical part of the phase diagram.