Cholesteric liquid crystals in rectangular microchannels: skyrmions and stripes

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 0.75 p , form bubble domain defects when the channel depth is around 0.75 p , 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 ∼2 p and the channel depth between 0.6 p to 1.1 p , 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. In this paper, we present experimental and numerical studies on the microstructures of a cholesteric liquid crystal (CLC) confined in rectangular micron-channels.