用偏光干涉法动态测量扭曲向列型液晶指向矢

提出一种基于双折射晶体劈的偏振态实时测量方法,并将该方法用于扭曲向列型液晶透射光的偏振态分析,实现了液晶分子平均指向矢的动态测量.利用基于晶体劈的偏光干涉法将待测光的偏振参量编码为两组干涉条纹,通过对干涉条纹定位实现偏振态的实时测量;再根据液晶指向矢的倾角、扭转角与透射光偏振态之间的关系,推算出液晶分子平均指向矢的动态信息.本实验测量液晶指向矢的速度为每秒5次,液晶平均指向矢的倾角、扭转角的测量准确度达到0.2°.本文研究为液晶分子平均指向矢的动态测量提供了一种有效途径.     

Influence of Surface Geometry of Grating Substrate on Director in Nematic Liquid Crystal Cell

The director in nematic liquid crystal cell with a weak anchoring grating substrate and a strong anchoring planar substrate is relative to the coordinates x and z. The influence of the surface geometry of the grating substrate in the cell on the director profile is numerically simulated using the two-dimensional finite-difference iterative method under the condition of one elastic constant approximation and zero driven voltage. The deepness of groove and the cell gap affect the distribution of director. For the relatively shallow groove and the relatively thick cell gap, the director is only dependent on the coordinate z. For the relatively deep groove and the relatively thin cell gap, the director must be dependent on the two coordinates x and z because of the increased elastic strain energy induced by the grating surface.     

Thermodielectric bistability in dual frequency nematic liquid crystal

We report on a thermodielectric bistability in dual frequency nematic liquid crystals (LCs) caused by the anisotropic nature of dielectric heating and director reorientation in an electric field. The bistability is a result of the positive feedback loop: director reorientation --> anisotropic dielectric heating --> dielectric anisotrophy --> director reorientation. We demonstrate both experimentally and theoretically that two states with different temperature and director orientation, namely, a cold planar state and a hot homeotropic state coexist in a LC cell for a certain frequency and amplitude range of the applied voltage.     

液晶孤子实验中指向角的时空分布

本文指出,从液晶透射的偏振光(单色光和白光)干涉条纹时空分布可以得到多种可能的液晶指向角时空分布。考虑具体物理条件(如指向切流孤子方程,非线性波局域速度随时间变化等)可以消除上述不确定性。文中对向列相切流液晶孤子情形做了详细讨论。数值计算结果与有关实验的比较表明,如果指向切流孤子方程(一维)是该实验的合理描述,那么,文献中指向角空间分布曲线是与该实验观察到白光下三条黑线速度方向相同这一事实相抵触的。本文给出了与上述实验比较一致的指向角分布。 关键词：     

Rayleigh–Benard Convection in the homeotropic nematic liquid crystal by a Gaussian laser beam absorption

The Rayleigh–Benard Convection (RBC) were studied theoretically in the homeotropic nematic liquid crystal (NLC) cell due to the absorption of a normally incident Gaussian light beam from upper side of the cell. It was shown that a convection of hydrodynamic motion in the NLC can be induced and eventually the molecular director coupling with hydrodynamics reorients the director. An external magnetic field parallel to the director decreases the director reorientation and the radial flow velocity, while the velocity in the z direction is unaffected. The results were found to be in a range that can easily be checked experimentally.     

Director deformation in a nematic liquid crystal with a ferromagnetic nanoparticle suspension

In view of the recent interest regarding nematic liquid crystals with a nanoparticle suspension, we investigate the director dynamics of a nematic matrix in the perspective of a soliton framework. The nematic liquid crystal is dispersed with ferromagnetic nanoparticles in order to enhance the director activity associated with the improved magnetization of the ferronematic matrix. The free energy density of the nematic matrix is deduced to be the Ginzburg–Landau (GL) equation for the director reorientation. We obtain a series of localized solutions for the GL equation, and also the dynamics is expressed in terms of the soliton evolution for the director by using the standard Hirota bilinearization procedure.     

Response of prestretched nematic elastomers to external fields

We investigate the response of prestretched nematic side-chain liquid single-crystal elastomers to superimposed external shear, electric, and magnetic fields of small amplitude. The prestretching direction is oriented perpendicular to the initial nematic director orientation, which enforces director reorientation. Furthermore, the shear plane contains the direction of prestretch. In this case, we obtain a strongly decreased effective shear modulus in the vicinity of the onset and the completion of the enforced director rotation. For the same regions, we find that it becomes comparatively easy to reorient the director by external electric and magnetic fields. These results were derived using conventional elasticity theory and its coupling to relative director-network rotations.     

Transversal structure of liquid crystal in a light wave field

The change in the orientation of the director of a nematic liquid crystal in a light wave field under a uniform illumination is analyzed. A system of equations for the distribution of the director angle of inclination is derived for the case of normal incidence. This system accounts for the finite energy of interaction between the nematic and substrate. The dependence of the director distribution profile on the incident light intensity and cohesion energy is studied at fields considerably above the threshold.     

Polarity-dependent dielectric torque in nematic liquid crystals

The dielectric dispersion in the uniaxial nematic liquid crystals affects the switching dynamics of the director, as the dielectric torque is determined by not only the present values of the electric field and director but also by their past values. We demonstrate that this "dielectric memory" leads to an unusual contribution to the dielectric torque that is linear in the present field and thus polarity sensitive. This torque can be used to accelerate the "switch-off" phase of director dynamics.     

Direct optical quantification of backflow in a 90 degrees twisted nematic cell

Optical guided mode observations of the transient director profile (optical tensor distribution) during the relaxation of a 90 degrees twisted nematic cell directly reveals backflow. In the first 6 ms of the relaxation process, after a voltage across the cell is removed, the midplane tilt of the director increases, reaching a maximum value of 101 degrees at 1.4 ms. This increase in midplane tilt is attributed to coupling between fluid flow (backflow) and director reorientation. A 270 degrees twisted state of the opposite handedness to the 90 degrees twisted state found at equilibrium is shown to exist during the backflow period. Good fits of theoretical models with experimentally determined time dependent director profiles yield the viscosity coefficients.     

Soliton-like molecular deformations in a nematic liquid crystal film

We study the nature of molecular deformations in a nematic liquid crystal film with elastic energy under homeotropic boundary conditions. The deformation in terms of splay, twist and bend fields of the director axis is found to be governed by the completely integrable Davey-Stewartson-I (DS-I) equation in (2+1) dimensions. Using the line soliton and breather solutions of the DS-I equation, the director axis is constructed, the components of which exhibit damped spatial oscillations. However, the splay and bend fields of the director axis exhibit localized structures of deformation.     

Orthogonal orientations for solvation of polymer molecules in smectic solvents

We explore the solvation of single conjugated polymer molecules in a smectic liquid crystal using single molecule spectroscopy. We find evidence for two distinct orientations for solvation: the majority of polymers are narrowly orientated parallel to the nematic director, as expected, but, unexpectedly, a second population is aligned perpendicular to the director. These molecules reside in the gaps between the layers. This latter type of solvation has not been previously observed and is not expected when the density variation along the director is sinusoidal.     

Fluid motion and molecular reorientation in a homeotropically orientated nematic liquid-crystal cell

The linearized Ericksen-Leslie differential equations, which couple fluid motion and director reorientation to each other, are reduced to a set of time varying differential equations for two pulsed optical waves incident at an angle upon a homeotropically orientated liquid-crystal cell. The differential equations are solved by a numerical method. The fluid velocity and the director angle are plotted as a function of space and time. It is shown that the reaction of fluid motion upon director reorientation is small.     

Nonlinear molecular deformations and solitons in a nematic liquid crystal

We studied nonlinear molecular deformations in a nematic liquid crystal with homeotropically aligned molecules and hard boundaries. As the basic dynamical equation for the director axis of the liquid crystal resembles the Landau-Lifshitz equation representing spin dynamics in a one dimensional classical continuum isotropic Heisenberg ferromagnetic spin chain, we invoke here the space curve formalism and the stereographic projection technique used in the case of the Heisenberg spin chain. Under space curve mapping, the director dynamics with elastic deformation is found to be governed by a perturbed nonlinear Schrödinger equation. A multiple-scale perturbation analysis brings out perturbed solitons to represent molecular deformations in the nematic liquid crystal. However, when a constant electric field is applied, the director dynamics is expressed under stereographic projection and the molecular deformations are found to be governed by periodic and localized static planar director configurations. A linear stability analysis on the static planar configurations shows that the system exhibits stable deformations.     

The inertia mode of the mechanically generated solitons in nematic liquid crystals

The existence of a localized relaxation mode of the recently observed solitarylike director waves in nematic liquid crystals is pointed out. This mode describes a nearly instantaneous regression of the perturbed director wave to its initial profile.Work supported by the Swiss National Science Foundation     

Director reorientation in twisted nematic liquid crystals due to thermomechanical effect

We have studied molecular director reorientation in a twisted nematic liquid crystal induced by a two-dimensional temperature gradient. We studied the effect of rate change between the temperature gradients in two directions. Our obtained director reorientations are in the range that can be observed experimentally very easily.     

Elastic energies for nematic elastomers

We discuss several elastic energies for nematic elastomers and their small strain expansions both in the regime of large director rotations, and in the case that director changes are small. We propose two fully non-linear model anisotropic energies and compare the behavior they predict with the currently available experimental evidence.     

Rotational reorientation of the director in twisted nematic liquid-crystal cells

The orientational relaxation of the director to its equilibrium orientation under electric, elastic, and viscous torques arising in twisted nematic liquid-crystal cells is investigated. It is shown that the relaxation time of the director depends strongly on the external electric field strength and weakly on the energy of anchoring of liquid-crystal molecules to the surfaces of the cell. The relaxation time of the director anomalously increases in electric fields close to the Fréedericksz threshold. It is established that, at specific strengths of the external electric field, the relaxation can occur in the form of traveling waves propagating from one edge to the other edge of the twisted nematic liquid-crystal cell. The calculations of the relaxation processes in the vicinity of the nematic-smectic A phase transition temperature demonstrate that the distortion of the director field is uniform over the entire cross section of the liquid-crystal cell and does not depend on the strength of anchoring of the liquid-crystal molecules to the surfaces of the cell.     

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.     

Electroconvection in nematics above the splay Fréedericksz transition

We present a basic model for an instability leading to a novel type of electroconvection patterns observed above the splay Fréedericksz transition in nematics. Such patterns, with wave vector perpendicular to the director easy axis, are found in planar sandwich cells under crossed polarizers, they do not produce shadowgraph images at onset. An adaptation of the classical Carr Helfrich mechanism is introduced. The ground state is a tilted director field uniform in the cell plane. The proposed mechanism destabilizes this director field and leads to a structure with modulated out-of-plane (twist) deformations. Experimental confirmation is provided by polarizing microscopy. All experimental observations are qualitatively explained with the proposed model.