Hard competition: stabilizing the elusive biaxial nematic phase in suspensions of colloidal particles with extreme lengths

TL;DR

This study uses simulations and theory to identify conditions under which stable biaxial nematic phases occur in colloidal particles with extreme aspect ratios, revealing new anisotropy thresholds and phase stability insights.

Contribution

It demonstrates the stabilization of biaxial nematic phases in hard polyhedral particles with high anisotropy, extending previous understanding of shape and size effects.

Findings

Stable biaxial nematic phases require high anisotropy and specific shape conditions.

Direct isotropic to biaxial nematic transition is prevented by intermediate phases.

Density functional theory confirms simulation results when including third virial coefficient.

Abstract

We use computer simulations to study the existence and stability of a biaxial nematic $N_b$ phase in systems of hard polyhedral cuboids, triangular prisms, and rhombic platelets, characterized by a long ($L$), medium ($M$), and short ($S$) particle axis. For all three shape families, we find stable $N_b$ states provided the shape is not only close to the so-called dual shape with $M = \sqrt{LS}$ but also sufficiently anisotropic with $L/S>9,11,14, 23$ for rhombi, prisms, and cuboids, respectively, corresponding to anisotropies not considered before. Surprisingly, a direct isotropic-$N_b$ transition does not occur in these systems due to a destabilization of $N_b$ by a smectic (for cuboids and prisms) or a columnar (for platelets) phase at small $L/S$, or by an intervening uniaxial nematic phase at large $L/S$. Our results are confirmed by a density functional theory provided the third virial coefficient is included and a continuous rather than a discrete (Zwanzig) set of particle orientations is taken into account.