FLEXOELECTRIC INSTABILITY IN NEMATIC LIQUID CRYSTAL BETWEEN COAXIAL CYLINDERS

Abstract

The stability of the equilibrium configurations of a nematic liquid crystal confined between two coaxial cylinders is analysed when a radial electric field is applied and the flexoelectric effect is taken into account. The threshold for perturbations depending only on the radius r in the cylindrical coordinate system and strong boundary conditions is studied. A new type of orientational transition caused by pure flexoelectric effect is found.     

固体电介质中的挠曲电效应

挠曲电效应通常描述为非均匀变形如应变梯度引起的电极化或者电场梯度引起的变形.应变梯度能够局部破坏晶体的反演对称从而在材料中诱导电极化,因此挠曲电效应是固体电介质材料中普遍存在的一种力电耦合效应.应变梯度和电场梯度均随材料尺寸的减小而迅速增大,在宏观尺度通常被忽略的挠曲电效应在微纳尺度反而起着非常重要的作用,会显著影响材料的物理性能.与压电效应和电致伸缩效应相比,挠曲电效应具有独特的尺寸依赖特征,其不受材料对称性和铁电材料居里相变温度的限制.论文综述了固体电介质中的挠曲电效应,并重点从理论、材料和应用方面综述了固体电介质中挠曲电效应的研究进展,对挠曲电效应的独特性能进行了详细地讨论,最后论文展望了固体电介质中挠曲电效应相关研究的开放性问题和发展方向.     

液晶表面物理——挠曲电效应研究及进展

向列液晶的“挠曲电效应”在1969年由Meyer首先提出,30年后这一理论才被重视.文章回顾了挠曲电效应的研究历程,总结了已取得的成果,并就挠曲电研究中的基础问题、研究进展、研究现状及存在的问题进行了系统的介绍,为从事此方面研究的工作者提供参考.     

Interaction between a +1 defect in a nematic thin film and the surface wall and flexoelectric response at the defect site

Within the Landau-de Gennes theory, we have investigated the interaction between a +1 defect whose nuclei is ‘circular’ nuclei (i.e. the director flux lines around the defect nuclei are circular) and the surface wall, using the one-dimensional finite-difference iterative method. +1 point defects will evolve into line defects perpendicular to the sample plane due to the existence of the surface wall. Therefore, we consider cylindrically symmetric solutions containing the line defect. The free energy of liquid crystal molecules around the defect nuclei and the surface energy produced by the surface wall are competing with each other. This competition makes polar angle of the director around the defect nuclei change considerably in a certain region, and the scale of this region will decrease with the surface anchoring strength increasing. On application of a DC electric field normal to the sample plane, the defect exhibits a flexoelectric response. We demonstrate that the field-driven structural changes at the defect site involve changes in both azimuthal and polar angles defining the local director.     

Numerical investigation of influence of ionic space charge and flexoelectric polarization on measurements of elastic constants in nematic liquid crystals

Deformations of nematic layers caused by magnetic field allow determination of the elastic constants of liquid crystal. In this paper, we simulated numerically the deformations of planar and homeotropic nematic layers. The flexoelectric properties of the nematic and presence of ions were taken into account. Our aim was to show the influence of flexoelectricity on the results of the real measurement of the elastic constants k₃₃ and k₁₁. In these simulations, we calculated the optical phase difference ΔΦ between the ordinary and extraordinary rays of light passing through the layer placed between crossed polarizers as a function of the magnetic field induction B. One of the elastic constants can be calculated from the magnetic field threshold for deformation. The ratio k₃₃/k₁₁ can be found by means of fitting theoretical ΔΦ(B) dependence to the experimental results. The calculations reveal that the flexoelectric properties influence the deformations induced by the external magnetic field. In the case of highly pure samples, this may lead to false results of measurement of the elastic constants ratio k₃₃/k₁₁. This influence can be reduced if the nematic material contains ions of sufficiently high concentration. These results show that the flexoelectric properties may play an important role, especially in well purified samples.