Local measurement of the zenithal anchoring strength

We present an electro-optic method for measuring the zenithal anchoring strength of nematic liquid crystals, based on the determination of the distortion produced by a small electric field. This method yields the zenithal anchoring strength at small applied torques, and remarkably, only needs local measurements (optical path difference versus applied voltage, sample thickness), in contrast to the classical methods that use measurements integrated over the entire sample. We determine the zenithal anchoring strength for two nematic liquid crystals (5 CB and 5 OCB) with positive dielectric anisotropy, onto poly(tetrafluoroethylene) (PTFE) treated surfaces, that yield planar liquid crystal cells. We find that the anchoring at the PTFE-5 CB interface is strong, with an extrapolation length approximately 30 nm, and independent of temperature far enough from the isotropic transition. We observe a pretransitional weakening of the anchoring strength near the nematic-isotropic transition, due to the reduction of the orientational order parameter at the interface. With 5 OCB, we measure a stronger anchoring, with an extrapolation length approximately 15 nm. This result may be explained by the increase of the van der Waals interactions between the liquid crystal molecules and the surface, due to the presence of the oxygen atom.     

Orientational transitions in a ferronematic layer with bistable anchoring at the boundary

A possibility of the first-order transition, as well as reentrant transitions, induced by an external magnetic field between the homeotropic phase and the hybrid homeotropically planar phase in a ferronematic liquid crystal (ferronematic) with bistable anchoring at the layer boundary is demonstrated in the framework of a continuum theory. The critical values of the material parameters of the ferronematic, the anchoring energy, the thickness of the layer, and the magnetic field strength, for which this transition is possible, are determined. The cases of positive and negative diamagnetic anisotropy of the ferronematic are considered.     

Study of intrinsic anchoring in nematic liquid crystals based on modified Gruhn-Hess pair potential

A nematic liquid crystal slab composed of N molecular layers is investigated using a simple cubic lattice model, based upon the molecular pair potential which is spatially anisotropic and dependent on elastic constants of liquid crystals. A perfect nematic order is assumed in the theoretical treatment, which means the orientation of the molecular long axis coincides with the director of liquid crystal and the total free energy equals to the total interaction energy. We present a modified Gruhn-Hess model, which is relative to the splay-bend elastic constant K₁₃. Furthermore, we have studied the free nematic interfacial behavior (intrinsic anchoring) by this model in the assumption of the perfect nematic order. We find that the preferred orientation at the free interface and the intrinsic anchoring strength change with the value of modification, and that the director profile can be determined by the competition of the intrinsic anchoring with external forces present in the system. Also we simulate the intrinsic anchoring at different temperatures using Monte Carlo method and the simulation results show that the intrinsic anchoring favors planar alignment and the free interface is more disordered than the bulk.     

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.     

Bistable surface anchoring and hysteresis of pitch jumps in a planar cholesteric liquid crystal

The relationship between bistable surface anchoring and the pitch jump process is examined for a planar cholesteric liquid crystal. Introducing a high-order, azimuthal surface anchoring potential into a simple model to describe a cholesteric, we derive an expression for the director twist as the natural pitch of the liquid crystal is allowed to vary. Writing the energy in terms of the surface twist, we are able to determine the twists which minimize the total energy of the system. We demonstrate how a pitch jump is related to an energy exchange from one branch of metastable states to another. We then discuss how the co-existence of energy minima and their associated solution branches may help explain the thermal hysteresis observed experimentally in cholesterics in the neighbourhood of a pitch jump. The presence of a higher-order surface energy term can expand the range of anchoring strengths in which pitch jumps are possible. We also investigate the influence of bidirectional surface anchoring on the behaviour of the total energy. Intermediate quarter-turn pitch jumps can occur, depending on the relative strength of the high-order anchoring term, and these can have a significant effect on the system hysteresis.     

Influences of surface and flexoelectric polarization on the effective anchoring energy in nematic liquid crystal

The physical effects on surface and flexoelectric polarization in a weak anchoring nematic liquid crystal cell are investigated systematically. We derive the analytic expressions of two effective anchoring energies for lower and upper substrates respectively as well as their effective anchoring strengths and corresponding tilt angles of effective easy direction.All of these quantities are relevant to the magnitudes of both two polarizations and the applied voltage U. Based on these expressions, the variations of effective anchoring strength and the tilt angle with the applied voltage are calculated for the fixed values of two polarizations. For an original weak anchoring hybrid aligned nematic cell, it may be equivalent to a planar cell for a small value of U and has a threshold voltage. The variation of reduced threshold voltage with reduced surface polarization strength is also calculated. The role of surface polarization is important without the adsorptive ions considered.     

Liquid crystal films on curved surfaces: An entropic sampling study

The confining effect of a spherical substrate inducing anchoring (normal to the surface) on rod-like liquid crystal molecules contained in a thin film spread over it has been investigated with regard to possible changes in the nature of the isotropic-to-nematic phase transition as the sample is cooled. The focus of these Monte Carlo simulations is to study the competing effects of the homeotropic anchoring due to the surface inducing orientational ordering in the radial direction and the inherent uniaxial order promoted by the intermolecular interactions. By adopting an entropic sampling procedure, we could investigate this transition with a high temperature precision, and we studied the effect of the surface anchoring strength on the phase diagram for a specifically chosen geometry. We find that there is a threshold anchoring strength of the surface below which uniaxial nematic phase results, and above which the isotropic fluid cools to a radially ordered nematic phase, besides of course expected changes in the phase transition temperature with the anchoring strength. In the vicinity of the threshold anchoring strength we observe a bistable region between these two structures, clearly brought out by the characteristics of the corresponding microstates constituting the entropic ensemble.     

液晶表面锚定能的温度依赖性研究

通过测量扭曲液晶盒中的液晶分子的实际扭曲角及摩擦方向与液晶分子的界面指向矢之间的方位偏角，用自行建立的液晶方位表面锚定能的测量方法，测定了５ＣＢ液晶与摩擦的聚酰亚胺界面的锚定能及其温度依赖性，并用热力学理论进行了分析．实验结果表明，随着温度的增加液晶的表面锚定能Ｅ_ａ是减小的，从２５.５℃时的５.０×１０^－5Ｊ／ｍ²减小到３５℃时的５.０×１０^－6Ｊ／ｍ²；同时，外推长度ｄ_e随 关键词：     

Exposure and temperature dependences of contact angle of liquid crystals on photoaligning surface

We report on first studies of wetting of liquid crystal on photoaligning surface. We observed strong light-induced variations of a contact angle of a liquid LC 5CB on a photoaligning surface of fluoro-polyvinyl-cinnamate (PVCN-F) and we connected these variations with changes of PVCN-F polarity. We also present drastic changes of the contact angle of a nematic liquid crystal (MLC-6080) on the PVCN-F surface in a vicinity of temperature anchoring transition of liquid crystal from homeotropic orientation to planar one. We did not find any peculiarities in temperature dependence of the contact angle of isotropic liquid (glycerol) on PVCN-F and of nematic liquid crystal on pure glass in the same temperature range. It allows us to suggest that rearrangement of LC molecules and flexible fragments in the LC-polymer interface are responsible for the change of surface tensions both of LC and polymer and the observed jump of the contact angle.     

Relation between the microscopic and macroscopic descriptions of the effect of a solid-substrate surface on a homeotropically aligned nematic liquid crystal

A simple microscopic mean-field model is proposed for a homeotropically aligned planar nematic liquid crystal (NLC) in contact with a solid-substrate surface. The intermolecular interaction in the NLC is simulated with the anisotropic McMillan potential, and the orienting effect of the substrate surface on the molecules in the NLC is described as that of an external field acting only on the first surface molecular layer of the sample. This model is used to describe the deformation of the director field of the sample caused by the external field and to determine the anchoring strength coefficient W, which is employed to macroscopically describe the orienting effect of the solid substrate on the NLC. The dependence of this coefficient on the strength of the short-range orienting field of the substrate surface used in the proposed microscopic model is found, and a unique correspondence between W and the profile of the orientational order parameter near the substrate surface is established. The temperature dependence of the anchoring strength coefficient is derived and found to agree well with experimental data for the MBBA NLC.     

Electric field-driven structural changes in cholesteric shells for optical applications

Cholesteric liquid crystal (CLC) shells with planar anchoring condition were prepared by using a capillary microfluidic technology. Under the influence of an electric field, shells were studied by filling them in glass cells coated with transparent ITO working as an electrode. CLC shells with a planar texture having dark brushes at 0 V under crossed polarizers transformed into a focal conic domain, a fingerprint texture, and finally to an isotropic phase with an increase of electric field that was confirmed by inserting the Bertrand lens in the polarized light path. In addition, our studies also show that the electric-field-driven isotropic phase was due to dielectric heating and local heating effects. The calculated magnitude of the local heating result from the model was in agreement with the experimental observation together with the chiral nematic to isotropic transition due to conventional heating.     

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.     

Surface driven transition in a nematic liquid crystal cell

Surface driven reorientation effects in a nematic liquid crystal cell caused by light-induced changes of the anchoring parameters were studied. Theoretical consideration of one-dimensional flat distributions of the director has shown that the director can undergo threshold reorientation between hybrid, homeotropic, and planar alignments as the anchoring energy varies continuously. The threshold reorientation takes place when the reference and light-induced easy axes are perpendicular. In the one-elastic-constant approximation the light-induced transition was found to be of second order as shown by a critical increase of the director thermal fluctuations in the vicinity of the transition point. These effects were experimentally studied in the cells containing 5CB liquid crystal aligned by the photosensitive azo-containing polymer layer. Zh. éksp. Teor. Fiz. 112, 2045–2055 (December 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Hydrodynamics of nematic liquid crystals due to direct volume expansion

Hydrodynamics of planar nematic liquid crystal molecules under the influence of horizontal pressure gradient was studied, theoretically. The pressure gradient is due to nematic liquid crystal volume expansion which is a consequence of the incompressibility of the liquid. The resulting director reorientation which is due to coupling between the director orientation and fluidity of the liquid is calculated numerically for the hard and weak anchoring conditions at the cell walls.     

Random anisotropy nematic model: Connection with experimental systems

We study theoretically the phase behavior of the continuum Random Anisotropy Nematic model. A domain-type pattern is assumed to appear in a distorted nematic liquid crystal (LC) phase. We map the model parameters to physical quantities characterizing LCs confined to Controlled-Pore Glasses and LC-aerosil dispersions. The domain size dependence on the disorder strength is obtained in accordance with the Imry-Ma prediction. The model estimates for temperature shifts of the paranematic-nematic phase transition and for the critical point, where this transition ceases to exist, are compared to the available experimental results.Received: 28 March 2004, Published online: 29 June 2004PACS: 61.30.-v Liquid crystals - 61.30.Dk Continuum models and theories of liquid crystal structure - 61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order - 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions     

丝状相液晶由表面相互作用产生的相变

导出双轴丝状相液晶在指向矢与易取向轴重合时的表面相互作用能.应用Ritz方法,对半无 界液晶体得出沿表面法向的双轴相(B_c)至沿表面某一轴的双轴相(B_a)的相变.随着表面锚泊强度双轴性部分的增大,B_a的产生机制逐渐由降低温度 转为表面锚泊,因此B_a相可出现在较高温度和较大温度范围内.在表面锚泊强度 单轴性部分α值的一定范围内,增大α值可使B_a在较高温度产生,但α值的进 一步增大则使液晶又从B_α返回B_c相. 关键词： 液晶 相变 表面能     

掺杂对液晶磁场弗里德里克斯转变影响的探讨

采用相位延迟法，通过对分别掺入微量卟啉类物质、纤维蛋白质和双偶氮苯沿面排列5CB液晶磁场弗里德里克斯转变的实验和数值计算，从实验和原理上探讨了微量掺杂对沿面排列5CB液晶磁场弗里德里克斯转变的影响.数值计算结果与实验相符，呈现两种转变过程.研究表明，掺杂物的磁学性质不同、分子的形状和大小不同对液晶弗里德里克斯转变和弗里德里克斯转变阈值的影响不同.进一步的掺杂2-benzothiazolethiol-linked porphyrinatozinc(Ⅱ)实验说明了这种情况. 关键词： 液晶 掺杂 弗里德里克斯转变     

Microscopic observations and simulations of Bloch walls in nematic thin films

We study Bloch wall defects formed by quenching nematic thin films from planar anchoring to homeotropic anchoring through a temperature-driven anchoring transition. The director profiles of the walls are directly visualized using fluorescence confocal polarizing microscopy, and shown to agree well with the simulation based on the Frank elasticity theory. A pure twist wall exists if the ratio of sample thickness to surface extrapolation length p is smaller than or close to 1; while a diffuse Bloch wall is obtained if p is much greater than 1.     

Surface electric-field-induced instabilities in a homeotropically oriented nematic liquid crystal sample

The effect of a surface electric field produced by ionic adsorption on the molecular orientation of a nematic liquid crystal sample is analyzed. The eigenvalue problem for a semi-infinite medium is analytically solved both for strong and weak anchoring situations. The threshold instabilities are numerically determined and it is shown that the homeotropic pattern can be destabilized also in the situation of strong anchoring. The dependence of the threshold field on the anchoring strength and on the surface polarization is determined by taking into account also the coupling of the quadrupolar component of the flexoelectric coefficient with the field gradient. Received 4 November 1999 and Received in final form 4 April 2000     

Electrically controlled local Frédericksz transition in a layer of a nematic liquid crystal

A local Frédericksz transition caused by electrically controlled surface anchoring in a nematic liquid crystal has been implemented owing to the displacement of surface-active ions. In the initial case, the nematic is homeotropically aligned by monomolecular layers of cetyltrimethylammonium bromide cations adsorbed on the surface of substrates. One of the substrates in a static electric field becomes free of a cation layer and specifies planar boundary conditions; as a result, a homeoplanar orientation structure is formed. The features of the dynamics of the optical response of the cell under study that are characteristic of the effect under consideration are discussed.