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     

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.     

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.     

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.     

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.     

Opto-acoustic modulator with a nematic liquid crystal

A liquid-crystal optical modulator working with acoustic signals is considered. It is shown that it can be operated also with white light up to frequencies of at least 1 MHz.     

Threshold property of a nematic liquid crystal cell with two grating surface substrates

A grating surface can drive the liquid crystal molecules to orientate along the direction parallel or vertical to the projected plane of the grating surface. The nematic liquid crystal (NLC) cell manufactured with two pre-treated grating surface substrates may realize the vertical display, parallel display and twist display. In this paper, the threshold property of this NLC cell is investigated systematically. With the Frank elastic theory and the equivalent anchoring energy formula of grating surface substrate, the analytic expressions of the threshold voltage related to three displays are obtained, which are dependent on their geometrical parameters such as amplitude δ and pitch λ of the grating surface substrate. For a certain anchoring strength, the threshold voltage increases or decreases with the increase of the value δ/λ of the different displays.     

Shear alignment behavior of nematic solutions induced by ultralong side-group liquid crystal polymers

Addition of a low concentration of a very long (430 kg/mol) side group liquid crystal polymer is shown to produce dramatic changes in the flow characteristics of a calamitic nematic liquid crystal. This polymer causes a typical flow-aligning nematic liquid crystal to align near the velocity gradient direction rather than near the velocity direction, corresponding to having a tumbling parameter lambda<-1, for concentrations greater than 7.5% polymer. Such flow-aligning behavior has not been reported previously in a calamitic nematic. The large molecular weight of the present polymer relative to those examined in the prior literature is responsible for these new phenomena.     

A biaxial nematic liquid crystal

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

Geometrical optics of a twist-oriented nematic liquid crystal

We have systematically analyzed the spatial dynamics of the polarization state of light in smoothly inhomogeneous anisotropic media. As a medium of this kind, we have considered a nematic liquid crystal with twist orientation. A system of coupled equations has been solved with respect to the Cartesian components of the electric-field component using a rotating coordinate system. We have shown that, upon propagation of light in this medium, the state of the polarization experiences oscillations the spatial frequency of which depends on the thickness of the specimen. Our analysis also has shown that, at an appropriate choice of the thickness, this twist cell behaves as a broadband half-wave plate.     

Behavior of block-polyampholytes near a charged surface

The behavior of polyampholytes near a charged planar surface is studied by means of Monte Carlo simulations. The investigated polyampholytes are overall electrically neutral and made up of oppositely charged units (called blocks) that are highly charged and of the the same length. The influence of block length and substrate's surface-charge-density on the adsorption behavior is addressed. A detailed structural study, including local monomer concentration, monomer mean height, and transversal chain size, is provided. It is demonstrated that adsorption is favored for long enough blocks and/or high enough Coulomb interface-ion couplings. By explicitly measuring the chain size in the bulk, it is shown that the charged interface induces either a swelling or a shrinkage of the transversal dimension of the chain depending, in a non trivial manner, on the block length.     

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.     

Viewing angle characteristics of a nematic liquid crystal cell with two crossed-grating surface substrates

Liquid crystal in a nematic liquid crystal cell surface with two crossed-grating surface substrates can be oriented along the normal grating direction with given geometric parameters of groove and anchoring strength. This display is equivalent to multi-domain vertical-alignment mode. It has a relatively wide viewing angle. In this letter, we investigate the viewing angle characteristics of this kind of cell. The viewing angle dependence of contrast ratio is obtained using the extended Jones matrix method, which also considers the flexoelectric effect. The viewing angle is dependent on the geometric parameter of grating surface, the flexoelectric coefficients, and the anchoring strength. Therefore, appropriate value for each factor needs to be selected to obtain a better viewing angle of this cell.     

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.     

Anisotropy of the turbidity of a nematic liquid crystal

The absorption coefficient of a nematic liquid present an important anisotropy. The measurement of this coefficient in three simple geometries allows the determination of the three Frank elastic constants on the same sample. We give the results obtained for MBBA at room temperature.     

Effect of an axial magnetic field on the Poiseuille flow of a nematic liquid crystal

The effect of an axial magnetic field on the Poiseuille flow of nematicp-azoxyanisole (PAA) has been computed using the Ericksen-Leslie continuum theory. The apparent viscosity decreases appreciably in the presence of the magnetic field. Orientation and velocity profiles for different shear rates and magnetic fields are presented.     

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.