Effect of shape biaxiality on the phase behavior of colloidal liquid-crystal monolayers

TL;DR

This study investigates how particle biaxiality influences the phase behavior of colloidal liquid-crystal monolayers, revealing that increased biaxiality generally destabilizes the biaxial nematic phase and introduces complex phase phenomena.

Contribution

It extends previous uniaxial particle models to include biaxial particles, providing detailed phase diagrams and insights into the stability of various nematic phases based on particle shape.

Findings

Biaxiality destabilizes the biaxial nematic phase in monolayers.

Reentrant uniaxial and biaxial phases occur for rod-like particles with high aspect ratios.

A density gap with stable biaxial nematic phase appears for very elongated particles.

Abstract

We extend our previous work on monolayers of uniaxial particles [J. Chem. Phys. 140, 204906 (2014)] to study the effect of particle biaxiality on the phase behavior of liquid-crystal monolayers. Particles are modelled as board-like hard bodies with three different edge lengths $\sigma_1\geq\sigma_2\geq\sigma_3$, and use is made of the restricted-orientation approximation (Zwanzig model). A density-functional formalism based on the fundamental-measure theory is used to calculate phase diagrams for a wide range of values of the largest aspect ratio ($\kappa_1=\sigma_1/\sigma_3\in[1,100]$). We find that particle biaxiality in general destabilizes the biaxial nematic phase already present in monolayers of uniaxial particles. While plate-like particles exhibit strong biaxial ordering, rod-like ones with $\kappa_1>21.34$ exhibit reentrant uniaxial and biaxial phases. As particle geometry is changed from uniaxial- to increasingly biaxial-rod-like, the region of biaxiality is reduced, eventually ending in a critical-end point. For $\kappa_1>60$, a density gap opens up in which the biaxial nematic phase is stable for any particle biaxiality. Regions of the phase diagram where packing-fraction inversion occurs (i.e. packing fraction is a decreasing function of density) are found. Our results are compared with the recent experimental studies on nematic phases of magnetic nanorods.