Cholesteric liquid crystals in rectangular microchannels: skyrmions and stripes

Abstract

In this paper, we present experimental and numerical studies on the microstructures of a cholesteric liquid crystal (CLC) confined in rectangular micron-channels. By using a sequence of microfabrication techniques we fabricated the micro-sized channels with accurately controlled size, aspect ratio and homeotropic surface anchoring. Through optical microscopic studies we established a phase diagram for the liquid crystal defect textures as a function of the channel depth and width. For the channel width larger than ∼2 times the cholesteric pitch p, the LC molecules are oriented primarily vertical to the channel when the channel depth is below 0.75p, form bubble domain defects when the channel depth is around 0.75p, and form stripe textures when the cell depth is above the cholesteric pitch p. In addition, the bubble domain size and the stripe texture periodicity are found to grow with the increase of the channel width. For the channel width smaller than ∼2p and the channel depth between 0.6p to 1.1p, no textures can be observed in the channels. Numerical simulations based on a director tensor relaxation approach yield detailed molecular director fields, and show that the bubble domain defects are baby-skyrmions and that the stripes are the first type of cholesteric fingerprints. A comparison with previous experiments and numerical simulations indicates that the size of the microchannels also influences what type of soliton-like topological textures form in the CLCs confined in the channels.

Computing equilibrium states of cholesteric liquid crystals in elliptical channels with deflation algorithms

“…The deflation technique plays a central role in the discovery of these disclination arrangements. For instance, numerical simulations in [39] relied on a priori knowledge, gained from experimental observations, that disclination structures should be present in order to initialise the Newton iterations within a basin of attraction. We emphasise that here the simulations are initialised with smooth director fields.…”

Comparing skyrmions and merons in chiral liquid crystals and magnets

“…(18) for the same L and ∆ E 2 is shown by the solid line in the same figure. These results are consistent up to a factor of 2, which is reasonable for such an approximate model. As noted in the Introduction, many experiments have studied skyrmions in confined cholesteric liquid crystals [4][5][6][7][8][9][10][11]. These experiments generally cannot determine whether skyrmions are metastable, as predicted by the calculations in this section, or whether they are actually stable structures.…”

“…In some cases, these defects are walls with a one-dimensional (1D) twist of the director field n(x) [3]. In other cases the defects are skyrmions, which have a 2D variation of the director field n(x, y) with double twist going outward from the center, covering all possible orientations on the unit sphere [4][5][6][7][8][9][10][11]. In even more complex cases, the defects are hopfions, with a 3D variation of the director field n(x, y, z) in a knotted texture [12,13].…”

Swelling Cholesteric Liquid Crystal Shells to Direct the Assembly of Particles at the Interface

“…The decreased number of adsorbed nanoparticles may allow disclination lines to anneal and become points of high twist, such as skyrmions. We believe these defects are skyrmions and not, for instance, torons because the stabilizing nanoparticles formerly followed planar anchoring, while torons do not have regions of planar anchoring surrounding them [31,32,33]. Other types of cholesteric structures are also stabilized under conditions of reduced nanoparticle concentration, where cholesteric fingers form alongside disclinations ( Fig.…”