Alternating defects and egg and dart textures in de-wetted stripes of discotic liquid crystal

Jonathan P Bramble,Daniel J Tate,Stephen D Evans,John E Lydon,Richard J Bushby

doi:10.1080/02678292.2021.1983048

Abstract

ABSTRACT Isotropic phase de-wetting of discotic liquid crystals on a surface patterned with alternating ~5-10 μm wide wetting and de-wetting stripes results in the formation of long narrow droplets. On slow cooling into the columnar phase, the liquid crystal aligns such that the columns lie either across or along the stripes. However, if the stripes are wider and/or the cooling rate is too fast, defects appear. When there are many such defects, the result is complex zigzag and wavy line optical textures, which are reminiscent of the egg and dart friezes associated with classical architecture. To a first approximation, all of these patterns can be seen as joined up fragments of developable domains in which the columns either circle a defect or propagate in a straight line. They are built up from motifs that involve bend but not splay or twist deformations; deformations that leave the two-dimensional lattice of the columnar phase unchanged. As is shown, these basic circular and straight-line motifs can be combined in a variety of different ways along the stripe but, in all of these, it is found that the defects alternate from side to side.

Highlights

Understanding how the director fields of liquid crystals (LCs) respond to confinement in thin films, fibres, dro plets etc. is fundamental to our use of these materials in displays, in optical compensating films [1], in the pro duction of high tensile strength fibres [2,3,4], and in chemical and biological sensors [5,6,7]
Whereas the discotic LC field has followed the prece dent set by the much-longer established calamitic LC field and has concentrated on the optical textures observed when samples are sandwiched between glass slides, in the case of discotic LCs, the textures observed for thin films, which are open-to-the-air can be much more informative [13]
When sandwiched between glass slides, columnar phases often anchor in a perpendicular manner so that the sample appears black

Summary

Introduction

Understanding how the director fields of liquid crystals (LCs) respond to confinement in thin films, fibres, dro plets etc. is fundamental to our use of these materials in displays, in optical compensating films [1], in the pro duction of high tensile strength fibres [2,3,4], and in chemical and biological sensors [5,6,7]. In the case of colum nar phases of discotic LCs, much of this behaviour is dominated by strong planar anchoring at an air inter face [9,10,11,12] and the fact that bend deformations are energetically much less costly than splay or twist defor mations of the director field. The behaviour of columnar phases is dominated by the for mation of ‘developable domains’ [13,14,15] Within these the columns propagate either in a straight line or along a circular path, Figure 1. This contrasts to the situation for lamellar phases of smectic LCs where behaviour in the bulk is dominated by the formation of Duplin cyclide/focal conic structures. The for mation of developable domains is driven by the need to

Methods

Results

Conclusion