Orientational changes in the nematic liquid crystal structure on a polymer surface induced by phase separation in a magnetic field

The change in the domain texture of the 5CB nematic liquid crystal on the surface of a polyvinyl butyral gelatinous polymer solution cured in a magnetic field is studied optically. The alignment of the nematic is described by the effective order parameter S* of the polydomain cells. The anchoring energy of a liquid crystal with gelatinous polymer W _ϕ = 1 × 10⁻³ erg/cm₂ is determined.     

Low-frequency electrodynamic instability in the smectic-A phase of a liquid crystal

An instability, induced by constant and alternating electric fields, is observed and studied in the planar texture of the smectic-A phase of a liquid crystal. The effect of the parameters of the applied field and of the material on the characteristics of the instability is studied. The mechanism for the appearance of the instability is linked to the nonuniform distribution of the electric field over the thickness of the liquid crystal.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 82–86, October, 1985In conclusion, the authors thank V. G. Chigrinov for useful discussion and interest in this work.     

Laser-induced helical structure in the isotropic phase of nematic liquid crystal

We report on the first observation of laser-induced helical structure in the isotropic phase of a nematic liquid crystal. The helical structure was induced by two nearly counter-propagating waves circularly polarized in the same sense. Bragg reflection in the optical tunneling region was studied. We made an extensive investigation on the analogy between Bragg reflection from a cholesteric liquid crystal slab and four-wave mixing. It provides a deeper insight into the physics of the optical properties of cholesteric liquid crystal.     

Quartic coupling and its effect on wetting behaviors in nematic liquid crystals

Based on the fact that rubbed groove patterns also affect the anchoring of liquid crystals at substrates,a quartic coupling is included in constructing the surface energy for a liquid crystal cell.The phase diagram and the wetting behaviors of the liquid crystal cell,bounded by surfactant-laden interfaces in a magnetic field perpendicular to the substrate are discussed by taking the quartic coupling into account.The nematic order increases at the surface while it decreases in the bulk as a result of the introduction of quartic substrate-liquid crystal coupling,indicating that the groove anchoring makes the liquid crystal molecules align more orderly near the substrate than away from it.This causes a different wetting behavior:complete wetting.     

Pyroelectric investigation of the surface polarization in the isotropic phase of a nematic liquid crystal

Measurements of the temperature dependence of the pyroelectric coefficients in cells with homeotropic and planar boundary conditions were performed in order to study the surface polarization and determine its nature in the isotropic phase of a nematic liquid crystal. Analysis of these data established that the ordering polarization arising as a result of the nonuniformity of the order parameter in the near-boundary region of the liquid crystal and the polar monolayer due to the bifilar properties of the molecules of the liquid crystal at the boundary with the solid wall of the cell are responsible for the surface polarization in the isotropic phase of a nematic liquid crystal. The values of the pyroelectric coefficients of monolayers and the coefficients of the ordering polarization for planar and homeotropic orientations are estimated.     

Wetting transition of a nematic liquid crystal on a periodic wedge-structured substrate

It is known that the wetting behaviour of a fluid is deeply altered by the presence of rough or structured substrates. We first review some simple considerations about isotropic fluids and rough substrates, and then we generalize Wenzel's law, which assigns an effective contact angle to a droplet on a rough substrate, when the wetting layer has an ordered phase, like a nematic. We estimate the conditions for which the wetting behavior of an ordered fluid can be qualitatively different from that usually found in a simple fluid. To support our general considerations, we use the Landau-de Gennes mean field approach to investigate theoretically and numerically the wetting transition of a nematic phase on a periodic triangular structured substrate.     

Thermal orientation effect in a nematic liquid crystal

The system of equations describing the thermal orientation effect in nonchiral liquid crystals, which was observed earlier, is proposed and substantiated. The effect of the director reorientation under the action of the temperature gradient is in many ways analogous to the Freedericksz transition in electric or magnetic fields. The angle of deviation of the director from the initial position is determined by the square of the temperature gradient. The effect is observed for a nematic liquid crystal, initially uniformly oriented, and is not accompanied by macroscopic fluxes of the medium under stationary conditions.     

向列相液晶中光学相位共轭技术的研究进展

对向列相液晶中光学相位共轭技术的研究进展进行了介绍,分析总结了向列相液晶中光学相位共轭过程的机理与实验方法,并对向列相液晶中的光学相位共轭技术的应用前景进行了展望。     

Effect of nonmesogenic impurities on the order of the nematic to smectic-A phase transition in liquid crystals

By means of adiabatic scanning calorimetry, we have studied the effect of the nonmesogenic solutes cyclohexane (CH) and biphenyl (BP) on the nematic-smectic-A phase transition in the liquid crystal octylcyanobiphenyl (8CB). For all concentrations of BP studied, the transition remains second-order. For 8CB+CH, however, crossover from second-order to first-order is observed at a tricritical point of the mole fraction x of CH around 0.0460. The difference between the two systems and the crossover is explained in terms of a mean-field free energy density expression including coupling terms of x with the nematic and smectic-A order parameters.     

Self-transparency effect in a twisted nematic liquid crystal cell

In a twisted nematic liquid crystal cell we report and study the self-transparency effect induced by the creation of an isotropic droplet. A simple theoretical model is presented and good agreement with experimental data is found. In a two beams configuration transistor-like behavior is observed and theoretically discussed.     

The interaction between a disclination in a nematic liquid crystal and a rubbed surface

For the case of a rubbed surface with a finite anisotropic surface tension, a model is developed for the structure and energy of a surface disclination and of an edge disclination near the surface. It is found that there is an energy barrier against detaching a disclination from the surface, and also that there is a critical distance within which a disclination is attracted, and beyond which it is repelled, by the surface. These properties may provide a measurement of the anisotropy of the surface tension.     

Dynamics of twist effect in a dual-frequency nematic liquid crystal

The dynamics of the electrooptical 90° twist effect in a dual-frequency nematic liquid crystal is investigated for wavelengths of 0.65 and 1.55μ m. It is shown that the boundary conditions of the interaction between the phases affect the optical threshold of the twist effect, the contrast, and the working voltage range. The switching time of the twist effect from the off to the on state upon a variation of the amplitude of a rectangular dc voltage pulse from 15 to 50 V changes from 1.5 to 0.3 ms for a thickness of the nematic crystal layer of about 7 μ m. The minimal time of switching from the “on” to the “off” state was 3 ms in the case when relaxation of molecules in a cell with asymmetric boundary conditions was controlled electrically. The dynamic range of transmittance variation at a wavelength of 1.55 μm extended to 30 dB.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.     

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.     

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     

Observation of polarization conflict caused by geometrical phase in a twisted nematic liquid crystal cell

We analyze the optical effects associated with an adiabatic rotation of a plane of polarization in a twisted nematic liquid crystal. The experimental verification was performed with a cell with linear rubbing of a front surface and circular rubbing of a rear surface. The expectations of the liquid crystal's orientation defect origin along the line of the maximum tension and a polarization conflict caused by geometrical phase are confirmed.     

The domain structure of a nematic liquid crystal layer in an oscillating poiseuille flow

A spatially periodic structure arising in a nematic liquid crystal layer with planar orientation under the effect of an oscillating Poiseuille flow is described theoretically. The effect is analyzed on the basis of hydrodynamic equations of nematic liquid crystals, from which a self-consistent set of equations for perturbations of hydrodynamic variables is separated. It is demonstrated that the structure type and the threshold parameters of the effect depend on the frequency and the layer thickness through the scaling combination ωh ². The dependence of the configuration of arising distortions on the value of viscosity α₃ is analyzed.