标题: | 胆固醇液晶盒中液晶结构转换机制的研究 Study on the Mechanisms of Texture Transitions in Cholesteric Liquid Crystal Cells |
作者: | 姚怡安 I-An Yao 王淑霞 吴俊杰 Shu-Hsia Chen Jin-Jei Wu 光电工程学系 |
关键字: | 胆固醇液晶;液晶结构转换;胆固醇相位光栅;Helfrich 形变;cholesteric liquid crystals;texture transition;cholesteric phase gratings;Helfrich deformation |
公开日期: | 2003 |
摘要: | 不论在基础科学或应用层面上,胆固醇液晶的结构转换都很引人入胜。为了设计胆固醇液晶元件的驱动方式,对于液晶结构转换机制的了解有其必要性。 在本论文中,我们主要探讨水平配向胆固醇液晶盒中,三种液晶结构转换的机制,其分别为homeotropic-planar液晶结构转换、homogeneous-planar液晶结构转换以及planar-focal conic液晶结构转换。另外,我们也探讨了垂直配向胆固醇液晶盒中,homeotropic-fingerprint液晶结构转换的机制。为了了解液晶结构转换的动态过程,我们根据有限原素法开发了一个三维指向矢模拟程式。 藉由上述的模拟程式,我们模拟了水平配向液晶盒中,当电场被瞬间关掉后,homeotropic-planar液晶结构转换的动态过程。此模拟结果已由实验得到验证。更进一步地,我们讨论偏压波形对homeotropic-planar液晶结构转换的影响。根据模拟与实验所得到的结果,我们设计了一个偏压波形,藉以缩短homeotropic-planar液晶结构转换的时间。同样地,我们也模拟当电场被瞬间关掉后,homogeneous-planar液晶结构转换的动态过程。模拟的结果符合了之前的实验观察。同时,我们也发现Helfrich形变不仅可藉由电场诱发,同时也可藉由弹性力诱发。因此我们比较了homeotropic-planar和homogeneous-planar液晶结构转换过程中,弹性力所诱发的Helfrich形变与planar-focal conic转换过程中,电场所诱发的Helfrich形变。另一方面,我们藉由改变电场来观察planar-focal conic液晶结构转换过程中所出现的周期性液晶结构。我们发现这些液晶结构要不是周期性的条纹液晶结构,不然就是周期性的六角形液晶结构,其中周期性的六角形液晶结构是第一次被实验观察到。 最后我们在具有图案电极的垂直配向液晶盒中,观察当电场被瞬间关掉后,homeotropic-fingerprint转换过程中所出现的条纹液晶结构。我们发现条纹的方向不仅和液晶盒厚度有关同时也和外加电场有关。在此实验中,我们制作出可随操作电场调变条纹方向的胆固醇液晶相位光栅,并可利用偏压来调变此相位光栅的间距。此外我们也提出了该元件的操作机制。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT008924510 http://hdl.handle.net/11536/78302 |
显示于类别: | Thesis |
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