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.     

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.     

液晶盒中挠曲电效应的计算

负性液晶MBBA在液晶盒中垂面排列,沿面外加电场后产生挠曲电效应.基于液晶弹性理论研究这一效应,基于变分理论得到了液晶指向矢满足的微分方程及边界条件.应用差分迭代方法进行了数值求解.在小角度近似下,得到了指向矢分布的解析解,与数值解和专著结果进行了比较.     

Theory of nine elastic constants of biaxial nematics

In this paper, a rotational invariant of interaction energy between two biaxial-shaped molecules is assumed and in the mean field approximation, nine elastic constants for simple distortion patterns in biaxial nematics are derived in terms of the thermal average （Dmn^（l）） （Dm＇n＇^（l＇））, where Dmn^（l） is the Wigner rotation matrix. In the lowest order terms, the elastic constants depend on coefficients Γ,Γ＇, λ, order parameters Q0 = Q0（D00^（2）） ＋Q2（D02^（2）＋D0-2^（2）） and Q2 = Q0（D20^（2）） ＋ Q2（D22^（2）＋D2-2^（2））. Here Γ and Γ＇ depend on the function form of molecular interaction energy vj′j″j （τ12） and probability function fk′k″k （τ12）, where r12 is the distance between two molecules, and λ is proportional to temperature. Q0 and Q2 are parameters related to multiple moments of molecules. Comparing these results with those obtained from Landau-de Gennes theory, we have obtained relationships between coefficients, order parameters used in both theories. In the special case of uniaxial nematics, both results are reduced to a degenerate case where K11=K33.     

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.     

挠曲电效应对向列相液晶盒响应时间的影响

应用液晶动力学理论和差分迭代方法,考虑上液晶挠曲电特性情形下研究强锚泊混合排列向列相液晶盒撤掉电压时的动态响应过程,数值计算得到液晶盒透过率随时间的变化.结果表明:随展曲和弯曲挠曲电系数和的绝对值|e₁₁+e₃₃|的增加,液晶盒的响应时间逐渐减小,当|e₁₁+e₃₃|=50 pC·m^-1时,响应时间与忽略挠曲电效应的情况相比缩短了一半.     

Laser light scattering technique for non-invasive<Emphasis Type="Italic">in situ</Emphasis> simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals

The laser light scattering technique for non-invasivein situ simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals is introduced. By measuring the autocorrelation function of the scattered light from nematic liquid crystals at different scattering angles, the splay and twist elastic constantsK ₁₁ andK ₂₂ are obtained from the amplitudes of the autocorrelation function, and the viscosity coefficients ofη _Splay andη _Twist are determined using the viscoelastic ratiosK ₁₁/η _Splay andK ₂₂ η _Twist from the telaxation parameters of the two modes.     

Structure,elasticity and phase transitions in liquid crystals with deformations

ABSTRACT

A molecular-statistical theory describing the nematic liquid crystals (LCs) with spherical inclusions (or point defects) is proposed. At given size of inclusions and nematic order parameters at the surfaces of inclusions (zero in the case of point defects) and far from inclusions (where the nematic LC is almost uniform), the distribution of nematic order parameters in the bulk of LC with inclusions was found to be fully determined by the elastic constants of LC. We have found and explained the two-step heat-driven transformation from the nematic phase into the isotropic phase, with the intermediate phase in between. The nematic order parameters and the elastic constants are evaluated in the framework of a unified approach based on the features of pair interaction potentials of the individual LC molecules. It is shown that, in the case of K₃₃ < K₁₁, the point defects should destroy the conventional nematic phase.     

Molecular theory of nematic liquid crystals viewed as effect of collective excitation in ferromagnetic systems

We develop a microscopic theory of the nematic phase with consideration of the effect of the collective excitation on properties of nematic liquid crystals. The model is based on the Heisenberg's exchange model of the ferromagnetic materials. Since the orientation of the molecular long axis and the angular momentum of the molecule rotating around its long axis have the same direction, operators can be introduced to research the nematic liquid crystals. Using the lattice model and the Holstein--Primakoff transformation, the Hamiltonian of the system can be obtained, which has the same form as that of the ferromagnetic substance. The relation between the order parameter and reduced temperature can be gotten. It is in good agreement with the experimental results in the low temperature region, the accordance is better than that of the molecular field theory and the computer simulation. In high temperature region close to the transition point, by considering the effect of the higher-order terms in the Hamiltonian, theoretical prediction is in better agreement with the experiment. That indicates the many-body effect is important to nematic liquid crystals.     

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.     

Flexoelectric origin of oblique-roll electrohydrodynamic instability in nematics

We develop the theory of electrohydrodynamic instability in nematic liquid crystals by incorporating the flexoelectric terms. Using a one-dimensional linear analysis of the problem for an applied DC field, we demonstrate that for the usual materials the rolls have an oblique orientation as has been found experimentally. We also provide an experimental evidence for the strong flexoelectric influence on the director profile in the rolls.     

Laser light scattering technique for non-invasive in situ simultaneous measurements on elastic constants and viscosity coefficients of nematic liquid crystals

Theelasticconstantsandtheviscositycoefficientsarethekeyparametersforthephysicalpropertiesofnematicliquidcrystals(LCs)[1,2],fortheyarecorrespondingtothethresholdvoltageandtherespondingtimeofliquidcrystaldisplaydevices[3,4].Sofarnotechniquethatcansimultaneouslymeasurethetwoparametersinsituhasbeenreported.TheelasticconstantsandviscositycoefficientsofnematicLCsatthepresentaremeasuredusingdifferentmethodsindependently.TheviscositycoefficientsisgenerallymeasuredwithOstwaldviscosimeter,andtheelasti…     

Influence of the molecular structure of components on the induction of mesophases in nematic liquid crystal systems

The dielectric properties of nematic liquid crystals (NLC) are studied in the temperature range of mesophase existence with taking into account the peculiarities of the molecular structure of components. The dependence of the ordering parameter and activation energy on the temperature and composition of an LC system are obtained and investigated by analysis of EPR spectra based on the spin relaxation theory. The values of the local viscosity of the systems are estimated within the framework of the hydrodynamic model. The appearance of induced and reentrant mesophases is established. It is shown that in NLC systems with non-center-symmetric molecules the correlation between the activation energy and thermostability should be inverse to that in the Maier-Saupe model.     

Theoretical analysis of the influence of flexoelectric effect on the defect site in nematic inversion walls

Based on the experimental phenomena of flexoelectric response at defect sites in nematic inversion walls conducted by Kumar et al., we gave the theoretical analysis using the Frank elastic theory. When a direct-current electric field normal to the plane of the substrate is applied to the parallel aligned nematic liquid crystal cell with weak anchoring, the rotation of ± 1 defects in the narrow inversion walls can be exhibited. The free energy of liquid crystal molecules around the +1 and-1 defect sites in the nematic inversion walls under the electric field was formulated and the electric-field-driven structural changes at the defect site characterized by polar and azimuthal angles of the local director were simulated. The results reveal that the deviation of azimuthal angle induced by flexoelectric effect are consistent with the switching of extinction brushes at the +1 and-1 defects obtained in the experiment conducted by Kumar et al.     

Convection in relatively thin layers of nematic liquid crystals

Laser-induced convective motion in thin layers (less than a millimeter) of nematic liquid crystals is investigated experimentally. It is shown that convection arising in the presence of the vertical gradient of temperature is toroidal. Velocities of convective motions in the liquid crystal are measured for two different diameters of the incident laser beam. Comparison is made of the threshold of the temperature gradient of convective motions obtained by theoretical estimations and with use of experimental parameters.     

Short range orientational order in nematic liquid crystals

The different approximations that have been used in applying Bethe’s cluster model to the nematic-isotropic phase transition are examined. It is shown that the introduction of a higher order term in the mean field potential of an outer molecule of the cluster improves the consistency of the theory considerably. In particular, the importance of satisfying Chang’s relation is emphasized. Calculations are presented of the long and short range order parameters, heat of transition and specific heat for different values ofz, the number of nearest neighbours around any given molecule, for both nonpolar and antiparallel near neighbour correlations. Even the new mean field potential appears to be inadequate forz=3.     

Electro-optic properties of nematic and ferroelectric liquid crystalline nanocolloids doped with partially reduced graphene oxide

Flakes of partially reduced graphene oxide (PRGO) were doped in nematic liquid crystals (NLCs) and ferroelectric liquid crystals (FLCs), respectively. The dielectric and electro-optical properties of NLCs doped with those flakes have been investigated. Threshold voltage and switching times are reduced by 30%–50%. This is primarily due to the decrease of the elastic properties of the nanocolloids compared to the non-doped nematics. The influence of the PRGO flakes on the spontaneous polarization, tilt angle and switching time of FLCs was investigated too. Such flakes reduce the response time by 40%–60%, increases spontaneous polarization by 20%–25% and increase the tilt angle by 15%–20%.     

表面液晶-垂直腔面发射激光器温度特性的研究

本文利用向列相液晶层作为激光偏振调控单元,涂覆于垂直腔面发射激光器（VCSEL）表面,测量并分析了不同温度下VCSEL正交线偏振光的阈值电流、峰值光功率和I-P特性. 实验结果表明：温度为293 K时,涂覆液晶后激光偏振第一跳变点和第二跳变点之间的电流值ΔI增大了2.2 mA,比无液晶时增大1倍. 温度为313 K、注入电流为3 mA时,两种正交线偏振光的光功率差ΔP由133.6 μW增大到248.8 μW,进一步增加了线偏振光的各向异性. 表面液晶层的引入有效地扩大了VCSEL的正交线偏振态稳定范围和光功率差,为实现液晶VCSEL高温单偏振稳定的设计和器件制备提供了理论和实验基础. 关键词： 垂直腔面发射激光器 向列相液晶 偏振态     

The elusive thermotropic biaxial nematic phase in rigid bent-core molecules

The biaxial nematic liquid crystalline phase was predicted several decades ago. Several vigorous attempts to find it in various systems resulted in mis-identifications. The results of X-ray diffraction and optical texture studies of the phases exhibited by rigid bent-core molecules derived from 2,5-bis-(p-hydroxyphenyl)-l,3,4-oxadiazole reveal that the biaxial nematic phase is formed by three compounds of this type. X-ray diffraction studies reveal that the nematic phase of these compounds has the achiral symmetry D_2h, in which the overall long axes of the molecules are oriented parallel to each other to define the major axis of the biaxial phase. The apex of the bent-cores defines the minor axis of this phase along which the planes containing the bent-cores of neighboring molecules are oriented parallel to each other.