Lassoing saddle splay and the geometrical control of topological defects

TL;DR

This paper demonstrates how the geometry of holes in nematic liquid crystals can be manipulated to control complex topological defect structures, using both experiments and simulations to explore saddle-like deformations and defect interactions.

Contribution

It introduces a method to induce and control saddle deformations and defect arrangements in nematic liquid crystals through hole geometry and anchoring conditions, advancing topological defect engineering.

Findings

Saddle deformations can be induced by hole geometry and anchoring conditions.

Defect structures such as disclination lines are tunable by elastic constants and geometry.

The study reveals potential for complex elastic deformation control in liquid crystal materials.

Abstract

Systems with holes, such as colloidal handlebodies and toroidal droplets, have been studied in the nematic liquid crystal (NLC) 4-cyano-4'-pentylbiphenyl (5CB): both point and ring topological defects can occur within each hole and around the system, while conserving the system's overall topological charge. However, what has not been fully appreciated is the ability to manipulate the hole geometry with homeotropic (perpendicular) anchoring conditions to induce complex, saddle-like deformations. We exploit this by creating an array of holes suspended in an NLC cell with oriented planar (parallel) anchoring at the cell boundaries. We study both 5CB and a binary mixture of bicyclohexane derivatives (CCN-47 and CCN-55). Through simulations and experiments, we study how the bulk saddle deformations of each hole interact to create novel defect structures, including an array of disclination lines, reminiscent of those found in liquid crystal blue phases. The line locations are tunable via the NLC elastic constants, the cell geometry, and the size and spacing of holes in the array. This research lays the groundwork for the control of complex elastic deformations of varying length scales via geometrical cues in materials that are renowned in the display industry for their stability and easy manipulability.