Unwinding of a cholesteric liquid crystal and bidirectional surface anchoring

TL;DR

This paper investigates how bidirectional surface anchoring affects the unwinding process of a cholesteric liquid crystal under a magnetic field, revealing complex transition behaviors and the influence of secondary easy axes.

Contribution

It introduces a model with high-order surface potential to analyze the impact of bidirectional anchoring on cholesteric unwinding, highlighting the role of secondary easy axes.

Findings

Transitions involve pitch jumps with quarter or half-turn twists.

Secondary easy axes significantly influence unwinding behavior.

Surface anchoring strength affects the transition pathways.

Abstract

We examine the influence of bidirectional anchoring on the unwinding of a planar cholesteric liquid crystal induced by the application of a magnetic field. We consider a liquid crystal layer confined between two plates with the helical axis perpendicular to the substrates. We fixed the director twist on one boundary and allow for bidirectional anchoring on the other by introducing a high-order surface potential. By minimizing the total free energy for the system, we investigate the untwisting of the cholesteric helix as the liquid crystal attempts to align with the magnetic field. The transitions between metastable states occur as a series of pitchjumps as the helix expels quarter or half-turn twists, depending on the relative sizes of the strength of the surface potential and the bidirectional anchoring. We show that secondary easy axis directions can play a significant role in the unwinding of the cholesteric in its transition towards a nematic, especially when the surface anchoring strength is large.