Biaxial nematic and smectic phases of parallel particles with different cross sections

TL;DR

This study uses a free-energy density functional approach to explore phase diagrams of biaxial particles with different cross sections, revealing conditions for stable nematic and smectic phases and the influence of particle geometry.

Contribution

It introduces a theoretical framework based on fundamental-measure theory to analyze the phase behavior of biaxial particles with varying cross sections, including the identification of a four-phase point.

Findings

Smectic phases with tetratic symmetry are metastable at low aspect ratios.

A four-phase point exists where multiple phase transitions converge.

Particle cross section geometry significantly influences phase stability.

Abstract

We have calculated the phase diagrams of one--component fluids made of five types of biaxial particles differing in their cross sections. The orientation of the principal particle axis is fixed in space, while the second axis is allowed to freely rotate. We have constructed a free-energy density functional based on fundamental--measure theory to study the relative stability of nematic and smectic phases with uniaxial, biaxial and tetratic symmetries. Minimization of the density functional allows us to study the phase behavior of the biaxial particles as a function of the cross-section geometry. For low values of the aspect ratio of the particle cross section, we obtain smectic phases with tetratic symmetry, although metastable with respect to the crystal, as our MC simulation study indicates. For large particle aspect ratios and in analogy with previous work [Phys. Chem. Chem. Phys. 5, 3700 (2003)], we have found a four--phase point where four spinodals, corresponding to phase transitions between phases with different symmetries, meet together. The location of this point is quite sensitive to particle cross section, which suggests that optimizing the particle geometry could be a useful criterion in the design of colloidal particles that can exhibit an increased stability of the biaxial nematic phase with respect to other competing phases with spatial order.