Dynamics in field-induced biaxial nematic liquid crystals of board-like particles

TL;DR

This study uses Dynamic Monte Carlo simulations to analyze the transport properties of field-induced biaxial nematic liquid crystals made of hard cuboids, revealing how particle shape influences diffusion in these complex phases.

Contribution

It provides the first detailed investigation of the equilibrium dynamics of field-induced biaxial nematic phases of monodisperse hard cuboids, highlighting shape-dependent diffusion behaviors.

Findings

Prolate cuboids diffuse faster in biaxial nematics than in less ordered phases.

Oblate cuboids do not show increased diffusion at high packing fractions.

Field-induced freezing and increased ordering affect particle mobility.

Abstract

Biaxial nematic ($N_B$) liquid crystals have been indicated as promising candidates for the design of next-generation displays with novel electro-optical properties and faster switching times. While at the molecular scale their existence is still under debate, experimental evidence, supported by theory and simulation, has unambiguously proved that suitable colloidal particles can indeed form $N_B$ fluids under specific conditions. While this discovery has sparked a widespread interest in the characterisation of the phase behaviour of $N_B$ liquid crystals, significantly less attention has been devoted to the study of their transport properties. To bridge this gap, by Dynamic Monte Carlo simulations we have investigated the equilibrium dynamics of field-induced $N_B$ phases comprising monodisperse hard cuboids. In particular, we calculated the long-time self-diffusion coefficients of cuboids over a wide range of anisotropies, spanning prolate to oblate geometries. Additionally, we have compared these diffusivities with those that, upon switching the external field off, are measured in the thermodynamically-stable isotropic or uniaxial nematic phases at the same density. Our results indicate that while prolate cuboids diffuse significantly faster in biaxial nematics than in less ordered fluids, we do not observe such an increase with oblate cuboids at high packing fractions. We show that these changes are most likely due to the field-induced freezing of the axes perpendicular to the nematic director, along with a substantial increase in the ordering of the resulting $N_B$ phase.