Monte Carlo simulation of binary mixtures of hard colloidal cuboids

TL;DR

This study uses Monte Carlo simulations to explore the phase behavior of colloidal hard cuboids, revealing that bidispersity and restricted orientations prevent the formation of biaxial nematic phases, contrary to some theoretical predictions.

Contribution

It demonstrates that bidisperse mixtures of hard cuboids do not form biaxial nematic phases and highlights the importance of considering positional order and full orientational freedom in phase behavior predictions.

Findings

Bidisperse HBPs do not form biaxial nematic phases.

Particles tend to phase separate into isotropic and smectic phases.

Limiting orientational degrees of freedom affects phase stability predictions.

Abstract

We perform extensive Monte Carlo simulations to investigate the phase behaviour of colloidal suspensions of hard board-like particles (HBPs). While theories restricting particle orientation or ignoring higher ordered phases suggest the existence of a stable biaxial nematic phase, our recent simulation results on monodisperse systems indicate that this is not necessarily the case, even for particle shapes exactly in between prolate and oblate geometries, usually referred to as self-dual shape. Motivated by the potentially striking impact of incorporating biaxial ordering into display applications, we extend our investigation to bidisperse mixtures of short and long HBPs and analyse whether size dispersity can further enrich the phase behaviour of HBPs, eventually destabilise positionally ordered phases and thus favour the formation of the biaxial nematic phase. Not only do our results indicate that bidisperse mixtures of self-dual shaped HBPs cannot self-assemble into biaxial nematic phases, but they also show that these particles are not able to form uniaxial nematic phases either. This surprising behaviour is also observed in monodisperse systems. Additionally, bidisperse HBPs tend to phase separate in coexisting isotropic and smectic phases or, at relatively large pressures, in a smectic phase of mostly short HBPs and a smectic phase of mostly long HBPs. We conclude that limiting the particle orientational degrees of freedom or neglecting the presence of positionally ordered (smectic, columnar and crystal) phases can dramatically alter the phase behaviour of HBPs and unrealistically enlarge the region of stability of the biaxial nematic phase.