Pressure dependence of the viscosities of nematic liquid crystals

The pressure dependence of a complete set of nematic viscosities is computed in the framework of a theory of the order parameter and temperature dependence of the nematic viscosities early proposed by us. The resulting equations are compared to the experimental data on the pressure dependence of the apparent viscosity measured in a falling ball experimental on MBBA, and the agreement found is good.     

Voltage-controlled optical switch in planar nematic liquid crystal film

This study presents an integrated device that consists of a directional coupler and an electro-optic switch. The device is designed to include a nematic liquid crystal cell, comprising a grating-like electrode. Applying the appropriate voltage to the cell yields a periodically distributed refractive index. An incident polarized beam will couple to an adjacent channel if it is parallel to the channel. The coupling efficiency is controlled by applied voltage. An obliquely injected polarized beam will be reflected and refracted in the channel, and propagated along a curved path. The route of the beam can be controlled by applying the voltage. A multiport routing was achieved for voltage modulation. In addition, the distribution of refractive index is also investigated by employing conoscopic technique experimentally and numerically.     

向列型液晶中的非局域孤子

 在两种极限情况下求得了向列型液晶中（1+1）D空间光孤子的精确解析解。对非局域非线性项作近似计算,获得了光束的演化方程。在弱非局域情况下,直接积分得出单峰孤子解;强非局域情况下,用贝塞尔函数表示明孤子的解析解,本征值个数与峰的个数一致,预示了多峰明孤子的存在;这些结果与其它文献的精确数值解一致。并把所得解与双曲近似解析解进行了比较。     

向列型液晶中的非局域孤子

在两种极限情况下求得了向列型液晶中（1+1）D空间光孤子的精确解析解。对非局域非线性项作近似计算,获得了光束的演化方程。在弱非局域情况下,直接积分得出单峰孤子解;强非局域情况下,用贝塞尔函数表示明孤子的解析解,本征值个数与峰的个数一致,预示了多峰明孤子的存在;这些结果与其它文献的精确数值解一致。并把所得解与双曲近似解析解进行了比较。     

First-order polarization-activated optical transitions in smecticC and planar nematic liquid crystals

Summary It is shown that polarization-activated intrinsic optical bistability (OB) may be obtained in smecticC or planar nematic liquid crystals without external feed-back resonator. First-order transitions for twist are proved to occur, in principle, irrespective of the liquid-crystal material constants. The light intensity, needed to obtain OB, ranges from 10 kW/cm² to 1 MW/cm². To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Theory of excluded volume effects in nematic liquid crystals

An expression for the free energy of nematic liquid crystals is given by taking account of the excluded volume effects as well as dispersion forces. The dependence of nematic order on concentration and/or shape of molecules is discussed.     

Laser-induced nonlinear thermal orientation effects in planar and homeotropic nematic liquid crystals

Nonlinear phenomena induced by laser in nematic liquid crystals are studied experimentally. It is shown that these phenomena are displayed in different forms depending on the initial orientation of liquid-crystal cell and laser polarization. The diffraction nature of the effects is eliminated experimentally and for their explanation the orientation-thermal mechanism is proposed.     

Electrically controlled surface diffraction gratings in nematic liquid crystals

Photorefractive diffraction gratings were studied in cells of homeotropically aligned pentyl-cyanobiphenyl liquid crystal. These holographic gratings were induced by the simultaneous and nonsimultaneous application of dc and coherent optical electric fields. The observed behavior was consistent with a predominantly surface-mediated photorefractive effect. Beam coupling was observed in all cases and led to a model involving screened and unscreened interfacial trapped charges driving a modulation of the easy axis. Holographic gratings could be switched on and off by the application of a small voltage.     

Determination of thermal instabilities thresholds for homeotropic and planar nematic liquid crystal samples

Thermal instabilities have been observed when a nematic liquid crystal sample is submitted to a temperature gradient. The value and the sign of the gradient threshold depend on the initial liquid crystal configuration. We calculate the gradient threshold for planar and homeotropic configurations by solving the exact two-dimensional problem taking into account the boundary conditions.     

Shear flow of a nematic liquid crystal near a charged surface

The maximum alignment angle θ_eff and the effective rotational viscosity coefficient γ _i ^eff of polar liquid crystals, such as 4-n-octyloxy-4′-cyanobiphenyl (8OCB), are investigated in the vicinity of charged bounding surfaces. The quantities θ_eff and γ ₁ ^eff are calculated in the framework of the Ericksen-Leslie theory. The results of calculations demonstrate that, for a homeotropic alignment of molecules on charged indium tin oxide surfaces, the effective rotational viscosity coefficient γ _i ^eff can increase by 7.8% as compared to the bulk rotational viscosity coefficient γ₁.     

Influence of barrier effects at interfaces on dynamic scattering of light in a nematic liquid crystal

The intensity of light dynamic scattering by a nematic liquid crystal with negative dielectric anisotropy (ZhK-440) as a function of the constant electric field strength has been studied under different interfacial conditions. It has been shown that potential barriers that arise at the interfaces not only influence the scattering intensity, but may also radically change the form of the dependence; i.e., the curves of optical transmission in the direction of the incident beam may have a minimum under certain field strengths. At higher strengths, the cells become totally transparent again. This anomalous behavior of the transmission curve is associated with the fact that the conductivity of the cells drops below a critical value with growing field strength, as the resistance of the space charge region at the inversely biased junction of the extraction electrode rises. In addition, it has been shown that the high resistance of the cells at low voltages may be attributed not only to a low concentration of impurities in the liquid crystal, but also to a low emissivity of the injecting electrode and a weak electron affinity of the extraction electrode.     

Rotational viscosity enhancement in nematic liquid crystal near a charged surface

The effective rotational viscosity coefficient and flow alignment angle are investigated for polar liquid crystals (LCs), such as 4-n-octyloxy- 4^′-cyanobiphenyl (8OCB), in the vicinity of a charged bounding surface. is calculated using the Ericksen-Leslie theory, both for stationary and nonstationary regimes. Calculations of , both for homeotropic and planar alignment of 8OCB molecules, at a charged indium tin oxide(ITO)-coated glass plate show an additional contribution to up to 7.8%. The nonequilibrium flow alignment angle (τ) is also calculated for the surface region bounded by 0.1≤y≤3.0 μm. Transition from a tumbling situation to a flow aligning regime can occur near the charged boundary surface. Received 22 November 2001 and Received in final form 31 January 2002     

Effect of surface electric field on the anchoring of nematic liquid crystals

We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We also discuss the director distortion when the anchoring energy of the surface is finite. Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997     

Pre- and post-transitional effects on the surface tension of nematic liquid crystals

The concept of a local order parameter is related to the existence of torque fields at a liquid crystal surface, and the principal thermodynamic quantities are determined.     

A new method for solid surface topographical studies using nematic liquid crystals

A new simple method has been developed to investigate the topography of a wide range of solid surfaces using nematic liquid crystals. Polarizing microscopy is employed. The usefulness of the method for detecting weak mechanical effects has been demonstrated. An application in criminology is foreseen.     

Optical determination of large distortion surface anchoring torques in a nematic liquid crystal

We have measured the angular dependence of the surface torque associated with a relatively weak anchoring of a nematic liquid crystal, for large deviations θ (near π/2) from the orientation of minimum energy. Analysing the torque in a Fourier series, we find that in addition to the usual first term, viz. sin 2θ, an important contribution from a sin 6θ term is needed to adequately represent the experimental variation.