Entropy-driven impurity-induced nematic-isotropic transition of liquid crystals

TL;DR

This study combines simulations and experiments to explore how impurities like water and hexane influence the nematic-isotropic phase transition in liquid crystals, revealing entropy-driven effects and impurity-specific behaviors.

Contribution

It introduces a combined experimental and simulation approach to understand impurity effects on liquid crystal phase behavior, highlighting the entropic origin of mixing-induced transitions.

Findings

Hexane induces a nematic-to-isotropic transition with concentration as control parameter.

Water causes only weak perturbations to nematic order.

A coarse-grained model explains the entropic nature of the phase transition.

Abstract

Phase behavior of liquid crystals is of long-standing interest due to numerous applications, with one of the key issues being how the presence of impurities affects the liquid crystalline order. Here we study the orientational order of 4-cyano-4$^{'}$-pentylbiphenyl (5CB) and 4-cyano-4$^{'}$-hexylbiphenyl (6CB) nematic liquid crystals in the presence of varying concentrations of water and n-hexane molecules serving as impurities, by carrying out both fully atomistic simulations and experiments. Our results reveal that mixing of the impurities (in case of hexane) with the host liquid crystals causes a nematic-to-isotropic phase transition with hexane concentration as the control parameter while demixing (in case of water) results in only weak impurity-induced perturbations to the nematic liquid-crystalline order. We develop a coarse-grained model illustrating the general nature and entropic origin of the mixing-induced phase transition.