空间各向异性两体势向列相液晶形变研究

基于分子两体势研究向列相液晶的形变.该两体势是空间各向异性的并且依赖于液晶的弹性常数.理论处理中假定具有理想向列序，这意味着分子长轴取向方向与液晶指向矢是重合的，而总自由能等于总相互作用能.以解析形式研究了三种基本的Fréedericksz 转变并对混合排列向列相液晶盒中的指向矢分布进行了数值计算.检查了文献中最近提出的两种从弹性能到两体作用势的映射方案，发现只有一种方案给出的结果与连续体理论一致. 关键词： 空间各向异性两体势 理想向列序 液晶形变 Fréedericksz 转变     

表面弹性项K13对液晶盒临界场强的影响和实验测定

以修正后的Rapini-Papoular公式为基础，得到了有表面弹性能K₁₃存在时，弱锚定向列液晶盒指向矢满足的方程和边界条件。用解析推导和数值计算的方法，详细研究了K₁₃对阈值场强和饱和场强的影响。在理论上给出了检验K₁₃是否存在的一种新方法及用沿面平行校列的弱锚定楔型液晶盒测定K₁₃值的实验方案，并通过数值实验证实了本文所给方法的可行性。     

表面极化对弱锚定向列液晶盒饱和特性的影响

以修正后的Rapini-Papoular锚定能公式为基础, 用理论分析和数值计算相结合的方法, 详细研究表面极化对液晶盒饱和特性的影响. 得出了求解指向矢分布的数学方程, 推导出了计算正常二级转变饱和电压的解析式, 同时结合最新报道, 给出了异常一级转变饱和电压的数值计算方法. 此外, 本文引入了反映指向矢分布特征的参量, 讨论了表面极化对此参量的影响. 结果表明, 指向矢最大倾角的位置随表面极化的增大将远离中央平面向基板移动. 一级转变饱和电压随弱锚定基板表面极化的增大而减小, 随强锚定基板表面极化的增大而增大. 而对二级转变, 饱和电压随表面极化的变化与一级转变恰恰相反. 本文所得结论对液晶表面物理的理解及液晶实际应用都具有重要的指导意义.     

基于修正的Gruhn-Hess两体势模型研究弯曲形变向列相液晶盒

使用修正的Gruhn-Hess空间各向异性两体势模型研究弯曲形变向列相液晶盒.为了易与弹性理论的结果进行比较,做单一弹性常数近似.在理想有序条件下,对模型进行连续化处理,截断到形变的二阶项,给出的自由能密度公式与含k₁₃项的弹性理论公式一致.使用模型直接研究强锚泊弯曲形变向列相盒,没有出现弹性理论给出的边界处不连续.通过Monte Carlo模拟研究发现,两体势中的k₁₃项将加剧弯曲盒中间层的涨落.     

挠曲电效应对向列相液晶盒电光效应的影响

研究挠曲电效应对向列相液晶盒电光效应的影响.通过变分理论得到液晶指向矢满足的微分方程和边界条件,利用差分迭代的方法计算强锚定和弱锚定液晶盒中液晶指向矢的分布情况.借助琼斯矩阵的方法,计算两种液晶盒电光特性曲线.此外,在计算中同时考虑了挠曲电表面和体积效应.     

表面序电极化、挠曲电极化与向列液晶盒饱和点的双稳态

理论分析和数值模拟相结合,研究了表面序电极化、挠曲电极化对非对称弱锚定向列液晶盒在饱和点双稳态的影响,得到了两种极化共存时液晶指向矢满足的方程和边界条件.通过引入新的状态参量,从能量的角度对两种极化的物理效应进行了分析.结果表明,在一定条件下,两种极化耦合,在饱和点通过改变锚定能影响边界条件,从而诱导双稳态.文中给出了判断是否存在双稳态的一般条件,此条件与数值计算结果符合. 关键词： 表面序电极化 挠曲电极化 饱和点 双稳态     

向列相液晶盒中的挠曲电体积效应

基于液晶弹性理论研究了电压作用下向列相液晶盒中的挠曲电体积效应.通过变分理论得到了液晶指向矢倾角θ满足的微分方程,利用差分迭代的方法计算了强锚定平行排列液晶盒和混合排列液晶盒中液晶指向矢的分布情况,利用琼斯矩阵的方法计算了两种液晶盒中电光特性曲线.结合计算结果分析了挠曲电体积效应对混合排列液晶盒中液晶指向矢分布及电光特性曲线的影响.     

液晶界面物理效应研究及进展

液晶表面和与它接触的固体表面的相互作用，作为边界条件对液晶显示起着重要的控制作用，并影响着液晶表现出的各种物理效应．液晶表面和界面科学一直是液晶物理学研究的重要领域．这一领域内一个重要的课题是，从物理机理出发寻找正确反映界面作用规律的数学物理方程，以便完善发展液晶界面理论，指导界面诱导技术．文章就这一研究领域已取得的成果、未解决的问题、当前研究的热点(一级转变和挠曲电效应)及目前有争议的问题(表面弹性项是否存在等)进行了介绍。     

弱锚泊对液晶微波相位调制的影响

液晶对微波的调制取决于外加电压作用下液晶分子的取向, 而基板的锚泊对液晶取向有重要影响, 必然导致微波调制的变化. 本文研究了无手性掺杂的弱锚泊90°扭曲向列相液晶的微波调制特性. 基于液晶弹性理论和变分原理得到了液晶盒系统的平衡态方程和边界条件, 采用差分迭代方法数值模拟了不同锚定强度大小和不同预倾角下单位长度相移随电压的变化. 结果表明: 1)预倾角对微波相移的影响与施加电压有关. 当液晶盒施加电压为0.5–1.6 V之间时, 随预倾角增大, 单位长度微波相移及其与强锚泊0°预倾角90°扭曲液晶相移差均增大, 且相移差达到最大时的电压值也随倾角增大而减小; 1.6–3.0 V之间, 单位长度微波相移及相移差随预倾角增大而减小; 1.6 V附近及3.0 V之后, 相移基本没有变化. 2)表面锚定强度大小对微波相移的影响非常大. 随锚定强度减小, 单位长度微波相移及相移差均会增大, 微波相移的可调范围也增大, 且增加越来越明显. 此研究为液晶微波调制器件的设计提供了理论依据.     

手性沿面排列IPS模式液晶盒中的动态响应

根据Ericksen和Leslie的液晶弹性理论和动力学理论对共面转换液晶显示器件(IPS LCDs)响应时间参数进行分析,将手性平行排列引入这种模式,给出此种盒中的指向矢分布的动态方程.通过计算液晶盒中的瞬时指向矢分布,得到响应时间参数与扭曲角之间的关系.计算结果表明:具有一定扭曲角的手性沿面排列IPS模式的液晶盒将会有很高的响应速度.     

First-order Fréedericksz transition at the threshold point for weak anchoring nematic liquid crystal cell under external electric and magnetic fields

Based on the modified formula of Rapini-Papoular, the equilibrium equation and boundary condition of the director have been obtained and the behaviour of the Fréedericksz transition at the threshold point has been studied for weak-anchoring nematic liquid crystal cells under external electric and magnetic fields with the methods of analytical derivation and numerical calculation. The results show that, except for the usual second-order transition, the first-order Fréedericksz transition can also be induced by a suitable surface anchoring technique for the liquid crystal cell given in the paper. The conditions for the existence of the first-order Fréedericksz transition are obtained. They are related to the material elastic coefficient k_{11}, k_{33} the thickness of the liquid crystal cell, the external electric field and the strength of surface anchoring, etc.     

Lowering of the Magnetic Fréedericksz transition threshold in nematic liquid crystals doped with ferromagnetic nanoparticles

We show that magnetic colloids could increase the coupling of a liquid crystal to a week magnetic field. Magnetic Fréedericksz transition in a planarly oriented nematic liquid crystal doped with spherical ferromagnetic nanoparticles is considered theoretically. It is shown that even spherical nanoparticles could decrease a threshold of this transition because they strongly increase the magnetic field in the medium. This phenomenon indicates the possibility to use the considered system in magneto-optical displays.     

Effect of an electric field on the orientation of a liquid crystal in a cell with a nonuniform director distribution

The electric field-induced reorientation of a nematic liquid crystal in cells with a planar helicoidal or a homeoplanar structure of a director field is studied theoretically and experimentally. The dependences of the capacitances of these systems on the voltage in an applied electric field below and above the Fréedericksz threshold are experimentally obtained and numerically calculated. The calculations use the director distribution in volume that is obtained by direct minimization of free energy at various voltages. The inhomogeneity of the electric field inside a cell is taken into account. The calculation results are shown to agree with the experimental data.     

Interface dynamics in liquid crystals

We have experimentally observed the pattern instabilities of an Ising wall formed in a nematic or cholesteric liquid crystal layer. We have deduced an envelope equation, relevant close to the Fréedericksz transition, from which we derived an equation for the dynamics of the interface in the vicinity of its bifurcation. In the case of the zig-zag instability, this model is characterized by a conservative and variational order parameter whose gradient satisfies a Cahn-Hilliard equation. We have also investigated the influence of slightly broken symmetries on the dynamical behaviour of the system. The disappearance of the interface translational invariance or of the reflection symmetry along the wall axis may induce new interfacial patterns which have been both experimentally and theoretically pointed out. Received 5 August 1999 and Received in final form 13 September 1999     

Fréedericksz deformation in nematic liquid crystals with frequency dependent dielectric constant

The rise and decay times of a Fréedericksz deformation in a planar nematic layer have been investigated. The liquid crystal used was an ester mixture with regimes of positive as well as negative dielectric anisotropy. This property allows to control both the rise and the decay times of the deformation by switching the frequency of the electric field. The kinetics was monitored with a fluorescent dye-probe.     

Saddle-splay elasticity and field induced soliton in nematics

The symmetry breaking Fréedericksz transitions, when a uniformly aligned nematic state is replaced by a homogeneously or periodically distorted state, have been extensively studied before. Here we analyse the influence of the saddle-splay elasticity on the non-linear ground state of a nematic liquid crystal in the presence of a magnetic field above the Fréedericksz threshold. We identify the bifurcation point when the localized soliton-like state is linearly unstable with respect to the perturbations of the wavevector in the direction perpendicular to the initial plane of the soliton. This instability occurs only if the ratio of the saddle-splay elastic constant to the elastic modulus of nematics in the one-constant approximation is above the critical value K24/K\ ≥0.707     

Thermomechanical effect in a planar nematic induced by a quasistatic electric field

A thermomechanical flow of uniformly oriented nematic liquid crystal induced by a quasistatic electric field is observed. This flow occurs when the electric field strength exceeds the static Fréedericksz transition threshold. The effect is attributed to an electric-field-induced nonuniformity of the director orientation which is required for the onset of the thermomechanical effect. The experimental results show good agreement with the theoretical estimates. Zh. Tekh. Fiz. 69, 122–124 (April 1999)     

Fréedericksz transition in confined liquid crystals: concentration and microgeometry effects

The Fréedericksz transition in dispersions of liquid-crystal droplets is studied analytically by balancing the electrostatic energy of the droplets with a strong anchoring elastic energy. Explicit dependence of the transition threshold field on the liquid-crystal volume fraction and the spatial arrangement of the droplets is obtained for the first time. As a result of the confined geometry, this threshold field does not depend on the thickness of the sample and splitting of the transition occurs in some situations.