Orientational ordering in thin interlayers of nematic liquid crystal 4,4'-penthyl-cyanobiphenyl

The European Physical Journal E - The method of polarization IR-spectroscopy was applied to investigate the change of structure of nematic liquid crystal 5CB interlayers between fluorite plates at...     

Orientational order and polarizability of molecules in a nematic liquid crystal

The influence of the orientational order of molecules in a nematic liquid crystal on the mean value $\bar \gamma$ and anisotropy Δγ of the molecular polarizability has been studied in the framework of the molecular statistical approach with allowance for the perturbation of the electronic structure of molecules due to the change in the conformation of their aromatic core and intermolecular interactions. Experimental dependences of $\bar \gamma$ and Δγ on the molecular orientational order parameter S have been derived, and their specific features for the known objects have been explained. The possibility of separating the contributions of opposite signs to the dependence Δγ(S) due to the change in the conformation of molecules and intermolecular interactions has been shown using nematic MBBA as an example.     

Molecular dynamics study of nematic liquid crystal orientation ordering

Molecular dynamics investigations of distribution functions of orientation, order parameters P ₂, P ₄ and selfdiffusion tensor components for nematic liquid crystal model were performed. Our results are in good agreement with experimental data for p-azoxyanisole (PAA). Applied molecular dynamics model is perspective for investigations of structure characteristics and transport coefficients of nematic phase.     

Orientational instability in a nematic liquid crystal in a decaying Poiseuille flow

The results of studies of orientational dynamics and instability in an MBBA nematic liquid crystal in a decaying Poiseuille flow are considered. The experiments were made on a wedge cell with a gap width varying in a direction perpendicular to the flow. Confining surfaces ensured homeotropic adhesion of the nematic to the surface. Above a certain critical value of the initial pressure drop, a uniform orientational instability is observed, which corresponds to the emergence of the director from the plane of the flow. The dependence of the critical pressure drop on the local thickness of the liquid crystal layer and on the external destabilizing electric field is determined. Simulation of nematodynamics equations is carried out. The results of theoretical calculations are in qualitative and quantitative agreement with the experimental data.     

Orientational dynamics in nematic liquid crystals

We examine the behaviour of single-particle orientational time correlation functions in nematic liquid crystals. As well as the expected dynamics involving oscillation in a mean-field potential, and occasional jumps between orientations parallel and antiparallel to the director, we provide the first simulation evidence of long-time tails characteristic of coupling to director fluctuations.     

Orientational optical nonlinearity induced by comb-shaped polymers in a nematic liquid crystal

The effect of optical orientation in nematic liquid crystals containing small additions of high-molecular compounds, i.e., comb-shaped polymers with light-absorbing azobenzene side fragments, was studied. The effects of light-induced reorientation of the director of nematic liquid crystals caused by light absorption of polymers and a low-molecular compound with a structure similar to side fragments of the polymers were compared in detail. An explanation was proposed for large values of the orientational nonlinearity induced by polymers.     

Surface-induced ordering in thin uniaxial liquid crystal films

The interface localization transition in thin uniaxial liquid crystal films with competing surface fields has been studied using Metropolis Monte Carlo simulations. The model is constructed from a lattice of continuously orientable interacting spins, and the Hamiltonian contains both bilinear and biquadratic contributions. The biquadratic contribution to the Hamiltonian is familiar from the Lebwohl-Lasher model, and accounts for the particle anisotropy in a liquid crystal. The head-tail asymmetry of the molecules in a uniaxial liquid crystal is taken into account through a bilinear contribution familiar from the classical ferromagnetic Heisenberg model with exchange anisotropy Lambda. The critical temperature T(c), characterizing the interface localization transition within the uniaxial liquid crystal film, depends strongly on the relative magnitudes of the bilinear and biquadratic interactions between the spins. For systems dominated by the biquadratic interaction, T(c) is found to be close to the bulk critical temperature of the system. But as the biquadratic interaction strength is reduced, T(c) departs markedly from the bulk critical temperature of the system.     

End-chain ordering in nematic liquid crystals

The ordering of hydrocarbon end-chains in nematic liquid crystals has been calculated by numerical summation over all chain configurations in the molecular field. The results, used in a self-consistent calculation of the orientational ordering for the full system, explain the variation of the isotropic-nematic transition temperatures (including the even-odd effect) and transition entropies with end-chain length.     

Cw thermal lens effect in thin layer of nematic liquid crystal

A cw laser induced, thermal lens effect in a thin layer of MBBA nematic liquid crystal was observed. Due to the birefringence both focusing and defocusing were obtained, depending on the polarization. A qualitative discussion of the aberrational effect in this thermal thin lens is given.     

Orientational phase transition and the solvation force in a nematic liquid crystal confined between inhomogeneous substrates

A nematic liquid crystal confined between two identical flat solid substrates, with an alternating stripe pattern of planar and homeotropic anchoring, is studied in the framework of the Frank-Oseen theory. By means of numerical minimization of the free energy functional we study the effect of the sample thickness D on the location of the phase transition between a uniform alignment, either planar or homeotropic, and a distorted nematic texture. The solvation force f due to distortions of the nematic director is also studied. It is found that f is always attractive, and for D small compared to the periodicity of the surface structure it exhibits two distinct asymptotic behaviors: f ∼ - D ^-1/2 or f ∼ - D ^-1, depending on the relation between D and the extrapolation lengths. Received 12 November 2002 Published online: 16 April 2003     

Nanostructures in a nematic liquid crystal

The effect of nanoobjects, molecular objects, and α particles introduced into a liquid-crystal matrix that is placed in a dc electric field is studied theoretically and experimentally. The introduced particles form a charged defect, the radius of which is much larger than the particle sizes, in the orientational structure of the liquid crystal. This defect is easily observed under an optical microscope, and its image agrees with calculated data. It is suggested that this effect be used for detection and diagnostics of small objects and for design of ionizing radiation detectors. The phenomena revealed in this work can help in producing new charged-defect-saturated liquid-crystal structures formed by introducing nanoparticles in a medium or by irradiating a medium by ionizing radiation. A nonmechanical principle of transport of nanoparticles and their regular incorporation into the lattice is implemented. The essence of this principle is control of molecular orientations in a liquid crystal by quasi-stationary electric fields.     

A biaxial nematic liquid crystal

We report a biaxial nematic liquid crystal in a simple thermotropic system.     

Orientational order parameter and rotational viscosity in C60 doped E7 nematic liquid crystal

The reorientational order parameter dependent complex linear susceptibilities are derived based on the angular averaged linear response theory of the molecular polarizability, which includes the Vuks type equations of anisotropic refractive indices and absorbances. The polar angular absorbance at 633 nm wavelength is measured as a function of the concentration of fullerene C₆₀ in E7 nematic liquid crystals (NLCs), from which the order parameter is deduced. It is shown that the order parameter has a linear dependence of dopant concentration. We also present a simple optical transmission method by adopting the closed-aperture Z-scan geometry under the influence of switching on and off the dc electric field to determine the optical relaxation time and the rotational viscosity coefficient of C₆₀ doped E7 NLC. The experimental results are pretty well explained by using the Osipov-Terentjev theory of the rotational viscosity with the measured order parameter.     

Orientational ordering and the effect of dimerization in wall-adjacent liquid layers

Comparative investigations of electronic-vibrational absorption spectra of thin layers of nonmesogenic liquids formed on dielectric and conductive substrates were carried out. Results are interpreted within the framework of Mayer–Saupe theory that takes into account the dispersion and the dipole–dipole interaction between molecules and molecular associates, and the influence of the field of surface forces as well.     

Orientational self-pumped phase conjugation of a light beam in a layer of a nematic liquid crystal with nonreciprocal feedback

Self-pumped phase conjugation of a Nd: YAG laser beam by forward stimulated orientational scattering in a nematic liquid crystal layer with a nonreciprocal feedback loop is investigated. It is found that the threshold of phase-conjugate wave generation is the lowest when the phase shift difference of pumping and scattering waves in the loop is /2.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 3–4, pp. 304–311, March–April, 1995.     

Orientational optical torque in a nematic liquid crystal,caused by trans- and cis-isomers of low- and high-molecular compounds

The effect of light on the conformational composition of low- and high-molecular (fifth-generation dendrimer) azobenzene dyes in a nematic host is studied. The orientational optical response of the nematic liquid crystal (NLC), caused by the presence of these compounds in the nematic host, is measured. The contributions of isomers to the orientational optical torque affecting the NLC director are determined. The results obtained suggest that the increase in the optical orientation efficiency in NLCs when using high-molecular dopants instead of low-molecular ones is unrelated to the difference in their conformational composition in the light field, but is caused by deceleration of orientational motion of chromophores.     

双轴性向列相液晶的相变理论

假设一双轴性向列相液晶分子简单相互作用模型，利用平均场理论，得到从各向同性相至单轴向列相、单轴向列相至双轴向列相的相变. 结果表明，在一定温度范围内，分子形状的非轴对称性可导致系统双轴向列相的产生. 在核磁共振实验中，导出双轴向列相序参数与谱线分裂频率间的一般关系，并讨论序参数测量性的可能性. 关键词： 向列相液晶 相变     

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。