Electric method for studying reorientation dynamics of the nematic liquid crystal director

A method has been proposed for studying the reorientation dynamics of the nematic liquid crystal (NLC) director using the results of measurements of the electric response of an LC cell. The simulation of the time dependences of the current in an LC cell with a homogeneous orientation is carried out upon variation of the applied voltage, the initial tilt angle of the director, dielectric anisotropy, and the elasticity coefficient, as well as the dynamic viscosity, density, and ion mobility in the NLC. A comparison of the experimental and computational curves of the electric response for NLC 5CB shows their good agreement. The method makes it possible to monitor the steady-state current, the density, and the ion mobility in NLCs.     

Buckling instability in liquid crystalline physical gels

In a nematic gel we observe a low-energy buckling deformation arising from soft and semisoft elastic modes. We prepare the self-assembled gel by dissolving a coil-side-group liquid-crystalline polymer-coil copolymer in a nematic liquid crystal. The gel has long network strands and a precisely tailored structure, making it ideal for studying nematic rubber elasticity. Under polarized optical microscopy we observe a striped texture that forms when gels uniformly aligned at 35 degrees C are cooled to room temperature. We model the instability using the molecular theory of nematic rubber elasticity, and the theory correctly captures the change in pitch length with sample thickness and polymer concentration. This buckling instability is a clear example of a low-energy deformation that arises in materials where polymer network strains are coupled to the director orientation.     

液晶的转动粘质系数的测试原理与方法

液晶的转动粘质度是液晶用于显示和通讯的重要物理参量.本文设计开发了基于微机的液晶参数测试光学系统，在该系统上研究了液晶的转动粘质系数的测试原理与方法：通过测试液晶盒在偏置电压下相位驰预时间，推导出转动粘质系数的值.测试Merck公司的 E7液晶的转动粘质系数随温度变化的曲线，进行了理论拟合，测试结果与理论符合得很好.通过分析另一系统对液晶介质常数随温度变化的曲线的测试结果，验证了该测试原理与方法的正确性.该测试方法具有简单、自动化的特点.     

Universal elasticity and fluctuations of nematic gels

We study elasticity of spontaneously orientationally ordered amorphous solids, characterized by a vanishing transverse shear modulus, as realized by nematic elastomers and gels. We show that local heterogeneities and elastic nonlinearities conspire to lead to anomalous nonlocal universal elasticity controlled by a nontrivial infrared fixed point. Namely, such solids are characterized by universal shear and bending moduli that, respectively, vanish and diverge at long scales, are universally incompressible, and exhibit a universal negative Poisson ratio and a non-Hookean elasticity down to arbitrarily low strains. Based on expansion about five dimensions, we argue that the nematic order is stable to thermal fluctuation and local heterogeneities down to d(lc)<3.     

Elastic energy-driven phase separation of phospholipid monolayers at the nematic liquid-crystal-aqueous interface

Experimental measurements and a thermodynamic model reveal that nematic elasticity can induce lateral phase separation of amphiphilic molecules assembled at interfaces between thermotropic liquid crystals (LCs) and immiscible aqueous phases. The morphologies of the phase-separated domains of amphiphiles induced by nematic elasticity are shown to be strongly dependent on the nature of the deformation of the LC. This study provides important insight into the physics that controls the ordering of molecules at interfaces of soft anisotropic materials, and identifies a new mechanism of phase separation at these interfaces.     

Flexoelectric polarization and second order elasticity for nematic liquid crystals

The flexoelectric properties of nematic liquid crystals are analysed. It is shown that in the frame of the usual elasticity two coefficients characterize completely the flexoelectric properties of the liquid crystal. These coefficients, in the limit of small scalar order parameter, i.e. near the clearing point, are approximately equal. More precisely they have the same linear term in the scalar order parameter, and differ for terms quadratic in this parameter. Their difference behaves, hence, as the usual nematic liquid crystal elastic constants, whereas their sum depends on the temperature, like the mixed splay-bend elastic constant. It is shown furthermore that in the frame of a second order elastic theory in the flexoelectric polarization there are no terms from the second order spatial derivatives of the nematic director, or the nematic tensor order parameter. Consequently also in the frame of the second order elasticity the flexoelectric polarization is given by the usual expression. This conclusion is important in connection with the surface polarization recently discovered in pretilted nematic liquid crystal samples.     

Effect of the network on the director fluctuations in a nematic side-group elastomer analysed by static and dynamic light scattering

Static and dynamic light scattering experiments were performed on monodomains of a crosslinked nematic side-group polysiloxane. The depolarized scattering caused by fluctuations of the nematic director was analysed. Due to a superposed strong static scattering produced by heterogeneities the fluctuating part of the scattering intensity was found to be rather small. In contrast to low molar mass nematics, the relaxation rates and strength did not show a significant dependence on the scattering vector. The results can be explained by restoring torques on the director originating from the coupling to the network elasticity which dominate those originating from the Frank elasticity. Quantities describing the coupling as well as effective viscosities for the director reorientation were obtained.     

Study of the Effect of Pulsed-Periodic Electric Field and Linearly Polarized Laser Radiation on the Properties of Liquid-Crystal Waveguide

Integrated-optical waveguides with a nematic liquid-crystal 4-cyano-4’-pentylbiphenyl (5CB) waveguiding layer have been investigated for different polarizations of incident laser radiation and under a pulsed-periodic electric field. A dependence of the damping coefficient of waveguide modes and the sizes of quasi-steady-state irregularities of nematic liquid-crystal layer on the linear polarization of laser radiation and the strength of pulsed-periodic field has been found experimentally. The correlation length is estimated for waveguiding layer irregularities. The waveguide scattering method has provided a resolution in correlation length exceeding the classical resolution limit by approximately an order of magnitude. The observed decrease in the damping coefficient of waveguide modes and irregularity sizes under external field is explained by the decrease in the correlation length of director fluctuations.     

Structure and elastic properties of a nematic liquid crystal: A theoretical treatment and molecular dynamics simulation

The Frank elasticity constants which describe splay (K ₁), twist (K ₂), and bend (K ₃) distortion modes are investigated for 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic liquid crystal. The calculations rest on statistical-mechanical approaches where the absolute values of K _i (i=1,2,3) are dependent on the direct correlation function (DCF) of the corresponding nematic state. The DCF was determined using the pair correlation function by solving the Ornstein-Zernike equation. The pair correlation function, in turn, was obtained from molecular dynamics (MD) trajectory. Three different approaches for calculations of the elasticity constants were employed based on different level of approximation about the orientational order and molecular correlations. The best agreement with experimental values of elasticity constants was obtained in a model where the full orientational distribution function was used. In addition we have investigated the approximation about spherical distribution of the intermolecular vectors in the nematic phase, often used in derivation of various mean-field theories and employed here for the construction of the DCF. We found that this assumption is not strictly valid, in particular a strong deviation from the isotropic distribution is observed for short intermolecular distances. Received 22 March 2000 and Received in final form 9 June 2000     

Some elastic properties of solid nematics

The general approach to study the properties of the mechanical deformations of solid nematics, which are the macroscopic homogeneous elastic media having the rotational symmetry of the nematic liquid crystals is proposed. The stress tensor, the Young modulus and the Poisson ratios for the parallel and perpendicular homogeneous orientations of nematic molecules relative to the axis of external forces influence are obtained by the varying of the free energy of mechanical deformation. It is shown that these constants have the anisotropic character and the experiments for the direct measurement of five elasticity coefficients entering the free energy expression are proposed.     

Cracks and topological defects in lyotropic nematic gels

We report on the effects of the coupling of nematic order and elasticity in anisotropic lyotropic gels consisting of large nematic domains of surfactant coated single wall carbon nanotubes embedded in a cross-linked N-isopropyl acrylamide polymer matrix. We observe the following striking features: (i) undulations and then cusping of the gel sidewalls, (ii) a nematic director field that evolves as the gel sidewalls deform, (iii) networks of surface cracks that are orthogonal to the nematic director field, and (iv) fissures at the sidewall cusps and associated topological defects that would not form in liquid nematics.     

Piezoelectricity of chiral nematic elastomers

A molecular model of freely jointed chains of chiral monomers is developed to describe the piezoelectric effect in chiral nematic elastomers. The model, an extension of the neo-classical theory of nematic polymer networks, takes into account a chiral biasing of molecular alignment under shear which leads to induced polarisation if the monomers contain a transverse dipole moment. The resulting theory is fully non-linear in elastic deformations, in the spirit of ordinary rubber elasticity. The expansion to the highest order in small strains gives the three linear piezoelectric coefficients predicted by phenomenological models. Received 7 September 1998 and Received in final form 19 October 1998     

Giant flexoelectricity of bent-core nematic liquid crystals

Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy.     

Phase change of a plane wave propagating through polymer dispersed liquid crystal film with nanosized droplets

The coherent field propagation in a polymer dispersed liquid crystal layer is described using a method based on the Foldy-Twersky integral equation for the vector case. Expressions for a polarization-independent phase shift and the coherent transmission coefficient of such a layer containing nanodimensional nematic liquid crystal droplets are obtained. Theoretical results for the phase shift are compared to the available experimental data.     

Isotropic-nematic interface and wetting in suspensions of colloidal platelets

We study interfacial phenomena in a colloidal dispersion of sterically stabilized gibbsite platelets, exhibiting coexisting isotropic and nematic phases separated by a sharp horizontal interface. The nematic phase wets a vertical glass wall and polarized light micrographs reveal homeotropic surface anchoring both at the free isotropic-nematic interface and at the wall. On the basis of complete wetting of the wall by the nematic phase, as found in our density functional calculations and computer simulations, we analyze the balance between Frank elasticity and surface anchoring near the contact line. Because of weak surface anchoring, the director field in the capillary rise region is uniform. From the measured rise (6 microm) of the meniscus at the wall we determine the isotropic-nematic surface tension to be 3 nN/m, in quantitative agreement with our theoretical and simulation results.     

Amplification of the emission of a liquid-crystal microlaser by means of a uniform liquid-crystal layer

The amplification of the emission of a microlaser based on a cholesteric liquid crystal by means of a uniformly oriented layer of a laser-dye-doped nematic liquid crystal is demonstrated. The nematic liquid crystal is characterized by the anisotropy of optical properties, including the amplification coefficient, which opens the possibility of creating amplifiers controlled by a low electric field.