Spinodal instabilities in polydisperse lyotropic nematics

TL;DR

This paper develops a generalized theoretical framework to analyze spinodal instabilities in polydisperse lyotropic nematic liquid crystals, revealing critical polydispersity thresholds and effects of doping on stability.

Contribution

It introduces a versatile model based on Onsager theory to locate spinodal points and analyze size fractionation in polydisperse nematics with arbitrary size distributions.

Findings

Both rod and disk nematics become unstable at ~20% polydispersity.

Doping with large species affects stability differently for rods and disks.

Large platelets suppress spinodal instabilities in discotic nematics.

Abstract

Many lyotropic liquid crystals are composed of mesogens that display a considerable spread in size or shape affecting their material properties and thermodynamics via various demixing and multi-phase coexistence scenarios. Starting from a generalized Onsager theory we formulate a generic framework that enables locating spinodal polydispersities as well as identifying the nature of incipient size fractionation for arbitrary model potentials and size distributions. We apply our theory to nematic phases of both hard rods and disks whose main particle dimension is described by a unimodal log-normal distribution. We find that both rod-based and discotic nematics become unstable at a critical polydispersity of about 20 %. We also investigate the effect of doping nematic assemblies with a small fraction of large species and highlight their effect on the stability of the uniform nematic fluid. We find that while rod-based are only weakly affected by the presence of large species, doping discotic nematics with very large platelets leads to a remarkable suppression of the spinodal instabilities. This could open up routes towards controlling the mechanical properties of nematic materials by manipulating the local stability of nematic fluid and its tendency to undergo fractionation-driven microphase separation.