Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels

TL;DR

This study investigates how liquid crystals confined in tiny silica channels undergo continuous paranematic-to-nematic transitions, differing from bulk behavior, with implications for understanding phase transitions in nanoconfined systems.

Contribution

It demonstrates that confinement induces continuous phase transitions in liquid crystals and links this behavior to wall-imposed ordering fields using a Landau-de Gennes model.

Findings

Continuous paranematic-to-nematic transition observed

Wall-imposed fields exceed critical threshold for continuous transition

Quenched disorder minimally affects transition temperatures

Abstract

The optical birefringence of rod-like nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 micrometer length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.