Waltzing route towards double-helix formation in cholesteric shells

TL;DR

This paper investigates complex defect structures in cholesteric liquid crystal shells, revealing new double-helix formations and transitions governed by shell geometry and confinement ratio, supported by experiments and theoretical modeling.

Contribution

It introduces a comprehensive study of defect structures in cholesteric shells, including the discovery of double-helix configurations and a theoretical framework explaining their formation.

Findings

Discovered double-helix defect structures in cholesteric shells.

Transition between configurations governed by confinement ratio c=h/p.

Observed defect waltz driven by chemical Lehmann effect.

Abstract

We study cholesteric order in liquid crystal shells with planar degenerate anchoring. We observe that the bipolar and radial configurations intensively reported for bulk droplets have a higher degree of complexity when the liquid crystal is confined to a spherical shell. The bipolar configuration is replaced by a structure where the boojums are linked to a stack of disclination rings that spans the shell, while the radial configuration exhibits a double helix structure where two disclinations wind around each other. Our results confirm recent numerical simulations and highlight the complexity of the defect structures arising when cholesteric liquid crystals are confined to spherical geometries. We also show that the position of the boojums is only ruled by the shell geometry, independently of the cholesteric pitch. To understand quantitatively this behavior, we develop a simple yet insightful theoretical framework which captures the essence of the observed phenomenology. We also show that the transition between the two configurations is solely governed by the confinement ratio c = h/p, where h is the average shell thickness and p is the cholesteric pitch. Finally, we perform a dynamical study of this transition, and report a fascinating defect waltz due to a chemical Lehmann effect.