Planar Nematic Liquid Crystal Research Articles

Theory of Rayleigh-Bénard convection in planar nematic liquid crystals.

A rigorous three-dimensional linear and weakly nonlinear analysis of Rayleigh-B\'enard convection in planarly aligned nematic liquid crystals is presented. We use realistic boundary conditions and allow for a stabilizing magnetic field. The analysis includes a determination of all parameters of the amplitude equation in different regimes of the magnetic field where normal, oblique, and parallel rolls occur. It turns out that there exists a range at intermediate fields were the primary transition is subcritical for the standard material (4-methoxybenzylidene-4'-n-butylaniline). The (``nonlinear'') stability of rolls is determined to second order in the amplitude so that nonpotential effects (especially mean flow) are included. In the low-field supercritical range the roll solutions are destabilized at a secondary threshold via a skewed-varicose instability leading presumably to spatiotemporal chaos. In the high-field supercritical normal-roll range a secondary transition can lead to oblique rolls that are ultimately destabilized by a short-wavelength instability.

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

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.

Optical-field-enhanced and static-field-induced first-order Fréedericksz transitions in a planar parallel nematic liquid crystal.

Optical-field-enhanced and static-field-induced first-order Fréedericksz transitions in a planar parallel nematic liquid crystal.

Magnetic Freedericksz Transitions of Nematics with Nonideal Initial Orientations

It is well known, that planar nematic liquid crystal cells with tilted director on the boundaries have no threshold field for magnetic Freedericksz transition. However, it is found that if one arranges the two glass plates of the cell such that the tilt angle of the director on one plate is opposite to that on the other, the same threshold magnetic field as in a cell with ideal initial orientation will be observed. It is found also that for a wedge-shaped cell with ideal initial orientation the threshold field will be lower than that for a planar cell.

ELECTROHYDRODYNAMIC INSTABILITIES OF NEMATIC LIQUID CRYSTALS——EFFECT OF AN INCLINED ELECTRIC FIELD

Electrohydrodynamic instabilities of a planar nematic liquid crystal (εαα>0) under an inclined electric field (at an angle θ0 with the normal of the glass plates of the cell) are considered. On the basis of the one-dimensional theory of Dubois-Violette et al, conditions for the onset of instabilities under the excitation of static, sinusoidal and square wave electric Fields, respectively, are obtained. Major results include: (1) Relaxation time of charge increases monotonically with θ0. (2) For MBBA, there exists a critical angle θc(ω)(ω is frequency of external field), when θ0> θc(ω) instability cannot occur. θc(0) =54.2°. (3) For MBBA, threshold voltage is a monotonic increasing function of θ0.

Formation of a smectic A phase from a strongly deformed planar nematic liquid crystal CBOOA

Formation of a smectic A phase from a strongly deformed planar nematic liquid crystal CBOOA

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.