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Effects of dielectric relaxation on director dynamics in uniaxial nematic liquid crystals.

机译：介电弛豫对单轴向列液晶中指向矢动力学的影响。

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摘要

Director reorientation dynamics is essential to most of today's liquid crystal technologies. Advances in research have allowed one to reduce the response times from millisecond to microsecond and even to nanoseconds. For these small time scales, the description of the director dynamics requires a proper account of the dielectric dispersion effects. However, the first analysis of the role of dielectric dispersion in the director dynamics has been performed only very recently. Previous investigations were limited to dual frequency nematic liquid crystals, and considered only a single dielectric relaxation process. In this thesis, we lift these limitations and propose a model that accounts for multiple processes of dielectric relaxations in both parallel and perpendicular dielectric permittivities. The model yields a number of counterintuitive predictions such as the polar contribution to the dielectric coupling between the field and the nematic liquid crystal. The most important results are summarized below:;First, we developed a general model to describe the director orientation dynamics of nematic liquid crystals under an applied electric field that accounts for multiple dielectric relaxations in both parallel and perpendicular dielectric permittivities. This model presents the contribution of dielectric relaxation as a "memory" effect; the director dynamics depends not only on the present electric and director fields, but also on their pre-histories.;Second, we discovered an unusual contribution to the dielectric torque as a result of dielectric memory effect. This torque is linear in the present applied electric field and is thus sensitive to its polarity. The phenomenon is accompanied by a spectacular but counter-intuitive effect: the director relaxation during the "switch-off" stage can be accelerated if instead of an abrupt vertical back edge of the voltage pulse, we apply a pulse with a non-instantaneously vanishing back edge.;Third, we demonstrate the director dynamics under different "shapes" of the front edge of the voltage pulses, taking into account the dielectric relaxation. Our model allows us to optimize the front edge of the pulse to improve switching efficiency.;Finally, we studied the dynamics of electrically induced isotropic-nematic (I-N) phase transition by accounting for the finite rate of the polarization dynamics, which has previously been considered an instantaneous process in the classic theory. Consequently the electrically induced nematic order parameter does not interact directly with the applied electric field, but is mediated by the dynamics of polarization. We present a model based on the Langevin equation that describes the dynamics of two crucial parameters: the polar and non-polar order parameters during the electrically induced I-N phase transition.
机译：导演重定向的动力学对当今大多数液晶技术至关重要。研究的进展已使人们能够将响应时间从毫秒减少到微秒，甚至减少到纳秒。对于这些小的时间尺度，对指向矢动力学的描述需要适当考虑介电色散效应。但是，对电介质色散在指向矢动力学中的作用的首次分析仅在最近才进行。先前的研究仅限于双频向列液晶，并且仅考虑了单个介电弛豫过程。在本文中，我们消除了这些限制，并提出了一个模型，该模型考虑了平行和垂直介电常数的介电弛豫的多个过程。该模型产生了许多违反直觉的预测，例如极性对场和向列液晶之间介电耦合的影响。最重要的结果总结如下：首先，我们开发了一个通用模型来描述向列液晶在施加电场下的指向矢取向动力学，该电场考虑了平行和垂直介电常数的多重介电弛豫。该模型将介电弛豫的贡献表示为“记忆”效应。导向器动力学不仅取决于当前的电场和导向器领域，还取决于它们的前历史。其次，由于介电记忆效应，我们发现了对介电转矩的不寻常贡献。该转矩在当前施加的电场中是线性的，因此对其极性敏感。这种现象伴随着一种壮观的但与直觉相反的效果：如果我们施加一个非瞬时消失的脉冲来代替电压脉冲的突然垂直后沿，则可以加速“关闭”阶段的指向矢松弛。第三，考虑到介电弛豫，我们演示了在电压脉冲的前边缘的不同“形状”下的指向矢动力学。我们的模型允许我们优化脉冲的前沿以提高开关效率。最后，我们通过考虑极化动力学的有限速率来研究电感应各向同性（IN）相变的动力学。被认为是经典理论中的一个瞬时过程。因此，电感应的向列级参数不直接与施加的电场相互作用，而是由极化动力学介导。我们提出了一个基于Langevin方程的模型，该模型描述了两个关键参数的动力学：电感应的I-N相变过程中的极性和非极性有序参数。

著录项

  • 作者

    Gu, Mingxia.;

  • 作者单位

    Kent State University.;

  • 授予单位 Kent State University.;
  • 学科 Chemistry Physical.
  • 学位 Ph.D.
  • 年度 2009
  • 页码 135 p.
  • 总页数 135
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 物理化学（理论化学）、化学物理学;
  • 关键词

  • 入库时间 2022-08-17 11:38:29

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