Uncovering different states of topological defects in schlieren textures of a nematic liquid crystal

Takuya Ohzono,Jun-Ichi Fukuda,Kaoru Katoh,Shigehiro Yamaguchi,Chenguang Wang,Aiko Fukazawa

doi:10.1038/s41598-017-16967-1

Abstract

Topological defects are ubiquitously found in physical systems and therefore have been an important research subject of not only condensed matter physics but also cosmology. However, their fine structures remain elusive because of the microscopic scales involved. In the case of a liquid crystal, optical microscopy, although routinely used for the identification of liquid crystal phases and associated defects, does not have resolution high enough to distinguish fine structures of topological defects. Here we show that polarised and fluorescence microscopy, with the aid of numerical calculations on the orientational order and resulting image distortions, can uncover the structural states of topological defects with strength m = ±1 in a thin cell of a nematic liquid crystal. Particularly, defects with m = +1 exhibit four different states arising from chiral symmetry breaking and up-down symmetry breaking. Our results demonstrate that optical microscopy is still a powerful tool to identify fine states of liquid crystalline defects.

Highlights

Liquid crystals (LCs)[1,2,3,4,5] have attracted interests of physicists as ideal model systems which facilitate direct observation of the structures of topological defects by optical means; for example, LCs have been studied as a tabletop model system of cosmic strings[6,7]
The head-tail symmetry of n reflects the apolar nature of the nematic liquid crystal (NLC), and the order parameter space of a two-dimensional (2D) NLC is S1/Z2, a circle (S1) whose two ends of a diameter are identical
We first investigate the basic properties of the anisotropic fluorescence emissions from the dyes, pyrromethene 597 (PMN) and c-Naphox (CNX)[37], in a uniaxially aligned NLC

Summary

Introduction

Liquid crystals (LCs)[1,2,3,4,5] have attracted interests of physicists as ideal model systems which facilitate direct observation of the structures of topological defects by optical means; for example, LCs have been studied as a tabletop model system of cosmic strings[6,7]. Combining experimental POM and FOM images with numerically calculated distributions of n and simple ray-tracing calculations to simulate the lensing effect due to the escaped structures, we uncover four different states of the defects with m = +1, attributable to the up/down symmetry breaking of the escape, and the chiral symmetry breaking of the distribution of n (Fig. 1e).

Results

Conclusion