Topological defects in liquid crystals
Topological defects in liquid crystals (LCs) dominate molecular alignment/motion in many cases. Here, the neural network (NN) function has been introduced to predict the LC orientation condition (orientation angle and order parameter) at local positions around topological defects from the phase/ polarization microscopic color images. The NN function was trained in advance by using the color information of an LC in a planar alignment cell for different orientation angles and temperatures. The photo-induced changes of LC molecules around topological defects observed by the time-resolved measurement were converted into the image sequences of the orientation angle and the order parameter change. It was found that each pair of brushes with different colors around topological defects showed different orientation angle and ordering changes.
The photo-induced change was triggered by the photo isomerization reaction of molecules, and one pair of brushes increased in its order parameter just after light irradiation, causing gradual rotation in the brush. The molecules in the other pair of brushes were disordered and rotated by the effect of the initially afected region. This combination approach of the time-resolved phase/polarization microscopy and the NN function can provide detailed information on the molecular alignment dynamics around the topological defects.
Topological defects in liquid crystals (LCs) have been paid much attention both in basic science and technological applications. They were also used for the analogical system for the cosmological theory1, 2. They are the structural singular points of the LC alignment, and there are a variety of defect types, which are categorized according to the topological charge. These topological defects can be spontaneously formed when LCs are put into a glass cell without an alignment layer. Due to the high elastic energy at these defect positions, they are thermodynamically intermediate states and will disappear for some duration of time. These days, the molecular alignment of LCs can be controlled by various techniques, and topological defects can be intentionally generated by the patterning of an alignment layer3–5. Many types of thin optics with topological defects using the LC patterns were demonstrated using a photo-alignment layer6–8. Topological defects also could be formed by stabilizing the alignment by a polymer pattern without a photo-alignment layer9.
Many interesting phenomena relevant to the topological defects were reported in biology, too. Te self-propelled bacteria’s motion was controlled by the LC patterns, including topological defects formed on a photo-alignment layer10. It was reported that the cell growth and the collective motion were controlled by the topological defects formed by the biological cells aligned like LC molecules, and the fate of the cells was determined by the type of the topological defects11, 12, and predesigned cell culture on a photo aligned LC elastomer was demonstrated13. Tose studies indicate that the collective motion of objects and molecules is dominated by topological defects, and the underlying principle should be studied.
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