Molecular field theory for biaxial smectic A liquid crystals

TL;DR

This paper develops a molecular field theory combining biaxial and smectic order parameters to predict phase diagrams of biaxial smectic A liquid crystals, revealing conditions for various phases and transitions.

Contribution

It introduces a two-parameter model integrating biaxial Maier-Saupe and McMillan theories, analyzing phase stability with two approximation methods.

Findings

Biaxial smectic A phase is predicted as the ground state for non-zero biaxiality and smecticity.

Rich phase diagrams include uniaxial and biaxial nematic and smectic phases with tricritical points.

The stability of biaxial nematic phases diminishes with increased smectic order, favoring biaxial smectic phases.

Abstract

Thermotropic biaxial nematic phases seem to be rare, but biaxial smectic A phases less so. Here we use molecular field theory to study a simple two-parameter model, with one parameter promoting a biaxial phase and the second promoting smecticity. The theory combines the biaxial Maier-Saupe and McMillan models. We use alternatively the Sonnet-Virga-Durand (SVD) and geometric mean approximations (GMA) to characterize molecular biaxiality by a single parameter. For non-zero smecticity and biaxiality, the model always predicts a ground state biaxial smectic A phase. For a low degree of smectic order, the phase diagram is very rich, predicting uniaxial and biaxial nematic and smectic phases, with in addition a variety of tricritical and tetracritical points. For higher degrees of smecticity, the region of stability of the biaxial nematic phase is restricted and eventually disappears, yielding to the biaxial smectic phase. Phase diagrams from the two alternative approximations for molecular biaxiality are similar, except inasmuch that SVD allows for a first order isotropic-nematic biaxial transition, whereas GMA predicts a Landau point separating isotropic and biaxial nematic phases. We speculate that the rarity of thermotropic biaxial nematic phases is partly a consequence of the presence of stabler analogous smectic phases.