Study on the Mechanisms of Texture Transitions in Cholesteric Liquid Crystal Cells
|關鍵字:||膽固醇液晶;液晶結構轉換;膽固醇相位光柵;Helfrich 形變;cholesteric liquid crystals;texture transition;cholesteric phase gratings;Helfrich deformation|
藉由上述的模擬程式，我們模擬了水平配向液晶盒中，當電場被瞬間關掉後，homeotropic-planar液晶結構轉換的動態過程。此模擬結果已由實驗得到驗證。更進一步地，我們討論偏壓波形對homeotropic-planar液晶結構轉換的影響。根據模擬與實驗所得到的結果，我們設計了一個偏壓波形，藉以縮短homeotropic-planar液晶結構轉換的時間。同樣地，我們也模擬當電場被瞬間關掉後，homogeneous-planar液晶結構轉換的動態過程。模擬的結果符合了之前的實驗觀察。同時，我們也發現Helfrich形變不僅可藉由電場誘發，同時也可藉由彈性力誘發。因此我們比較了homeotropic-planar和homogeneous-planar液晶結構轉換過程中，彈性力所誘發的Helfrich形變與planar-focal conic轉換過程中，電場所誘發的Helfrich形變。另一方面，我們藉由改變電場來觀察planar-focal conic液晶結構轉換過程中所出現的週期性液晶結構。我們發現這些液晶結構要不是週期性的條紋液晶結構，不然就是週期性的六角形液晶結構，其中週期性的六角形液晶結構是第一次被實驗觀察到。
The texture transitions in cholesteric liquid crystals are very interesting and are of importance for both fundamental science and applications. It is essential to understand the mechanisms of texture transitions in ordr to design driving schemes for cholesteric devices. In this thesis, we investigated the mechanisms of three types of texture transitions in planar-aligned cholesteric liquid crystal cells: homeotropic-planar texture transition, homogeneous-planar texture transition and planar-focal conic texture transition. In addition, the mechanism of homeotropic-fingerprint texture transition was discussed in homeotropic-aligned cholesteric liquid crystal cells. In order to understand the dynamics of texture transitions, a computer program for three-dimensional simulation based on the finite element method was developed. When the applied electric field was turned off abruptly, the dynamics of homeotropic-planar texture transition was numerically analyzed and experimentally confirmed in a planar-aligned cholesteric liquid crystal cell. Furthermore, the effect of bias waveform on the homeotropic-planar relaxation process was also studied. On the basis of this knowledge, a bias waveform was designed to reduce the long relaxation time. Similarly, we also numerically investigated the dynamics of the homogeneous-planar texture transition when the unwound electric field was removed abruptly. The simulation results agreed well with the previous experimental observations. Moreover, we found that the Helfrich deformation can be induced not only by an electric field but also by an elastic force. Therefore, we compared the elastic-induced Helfrich deformation during the homeotropic-planar and homogeneous-planar texture transitions with the electric-induced Helfrich deformation during the planar-focal conic texture transition. On the other hand, we observed the modulated textures during the planar-focal conic texture transition by changing the applied electric field. It was found that the modulated textures exhibit either an ordered striped texture or an ordered hexagonal texture depending on the applied electric-field strength. Of these textures, the ordered hexagonal texture was experimentally observed for the first time. Finally, we observed the stripe formation during the homeotropic-fingerprint texture transition when the applied electric field was turned off abruptly in homeotropic-aligned cholesteric liquid crystal cells with patterned electrode configurations. The striped direction depended not only on the thickness-to-pitch ratio, but also on the applied electric field. In this work, the cholesteric liquid crystal phase grating with the field-controllable grating orientation and grating period was realized and the operational mechanism of this device was presented.
|Appears in Collections:||Thesis|