Non-linear dielectric effect in the isotropic phase of antiferroelectric liquid crystals

We have theoretically studied the non-linear dielectric effect in the isotropic phase of antiferroelectric liquid crystals on the basis of phenomenological theory. We find an analytical expression for the non-linear dielectric effect in the isotropic phase of the transition. The temperature dependence of the non-linear dielectric effect is presented in the isotropic phase of the transition.     

Influence of pressure on the electric-field-induced phase transitions in liquid crystals

Within the framework of Landau-de-Gennes formulation, we analyse the effect of pressure on electric-field-induced phase transitions in a liquid crystal which shows spontaneously an isotropic-smectic A transition. Inferring from the experimental pressure dependences on the layer spacing in smectic A phase, as well as the nematic-smectic A metastable temperature T^*_AN, we incorporated the pressure dependence in the free energy through (the surface energy term) and the coupling between the quadrupolar nematic ordering Q_ij and the smectic order parameter ψ. From the S-T phase diagram, we found that the stability of field-induced nematic phase increases with pressure, whereas the discontinuity of the transition decreases. Also, the region where paranematic phase transits directly to smectic A phase increases with pressure.     

Surface transitions in nematic liquid crystals oriented with Langmuir-Blodgett films

In this paper two types of surface transitions in nematic liquid crystals are theoretically analyzed with a simple model in which the anchoring energy is the result of both steric and electric interactions. The physical model is proper to explain both the transition temperature experimentally observed in homeotropically aligned nematic liquid crystals, and the change in the mean molecular direction due to the variation of the number of surfactant films, piled on the boundary surfaces with the Langmuir-Blodgett technique.Partially supported by the TEMPUS program     

A phenomenological theory of the isotropic to chiral smectic-C phase transition

In this paper we discuss the direct isotropic to chiral smectic-C phase transition on the basis of a phenomenological theory. The model free energy is written in terms of the coupled order parameters including the spontaneous polarization. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a direct isotropic to chiral smectic-C phase transition is possible when compared to other phase transitions. On the basis of this model the isotropic-smectic-C^* transition is always of first order. The theoretical predictions are compared with the available experimental results.-1     

Pressure variation of reentrant transition temperature in liquid crystals

High pressure experimental studies show that in certain mesogenic materials, the nematic-smectic A (N-Sm A) transition temperature T_AN exhibits nonlinear pressure dependence. As a consequence, the material shows reentrant phenomena that is a phase sequence nematic — smectic A — reentrant nematic appears. The characteristic features of this phenomenon have been addressed here within the framework of Landau-de-Gennes theory, where the coupling between nematic and smectic A order parameters (γ, λ_eff) plays an important role. The cubic coupling γ is chosen to be negative in order to form Sm A phase whereas the biquadratic coupling λ_eff is made large and positive to obtain reentrant behaviour. In the present work, we incorporate the pressure dependence in the theory through γ and λ_eff which justifies the experimental pressure dependence in the reentrant transition temperature . The pressure dependence of γ and λ_eff are employed in the calculation of excess specific heat capacity near the reentrant transition. The computed heat capacity shows strong pressure dependence near the reentrant transition which can be confirmed from high pressure measurement.     

Magnetically induced Freedericksz transition and relaxation phenomena in nematic liquid crystals doped with azo-dyes

A static and dynamic investigation was performed on liquid crystal cells containing pure nematics and nematics doped with an azo-dye (Methyl Orange). It was found that the critical field for magnetic Freedericksz transition was decreased in samples containing the “trans” isomer and increased in those containing the “cis  ” isomer. Changes in the relaxation time _τA${\tau }_A$, _τB${\tau }_B$ intervening when switching on/off the magnetic field were also noticed. A theoretical model was elaborated to explain these phenomena.     

Universal growth laws in liquid crystals far from equilibrium

Universal growth laws were examined experimentally for the transition from an isotropic melt to the liquid-crystalline state as well as the liquid-crystal (LC) to crystal transition for a system, which can be largely super-cooled. For large quench depths the growth exponent of the growth law L(t) ∼tⁿ is given by n=1. On decreasing the quench depth, two phase-ordering processes can be resolved for the isotropic (Iso.)–LC transition, one with a decreasing growth exponent 1<n<1/2 and a long-time process with n=1, independent of quench depth. In the very vicinity of the transition, nucleus growth is described by a single process according to L(t) ∼t^1/2. This behavior is interpreted in terms of an Iso. to blue-phase (BP) to cholesteric (N*) transition. The crystallization from the liquid-crystalline state (monotropic smectic A*) can be super-cooled substantially and follows a linear-growth process L(t)∼t. Received: 3 July 2000 / Accepted: 17 October 2000 / Published online: 10 January 2001     

Geometry of nematic liquid crystals under shearing flow

In this work, an extended version of the Hess-Baalss conformal approach is used to propose a relation connecting the viscosity coefficients of the nematic liquid crystals. Starting from the well-known fact that, in its usual form, the conformal transformation leads to results which are not observed experimentally, it is shown that, when the director field of an ordered nematic phase under sheared motion is taken as a three-dimensional surface with torsion, the resulting theory describes the observed experimental data efficiently. Moreover, this model predicts that the five viscosity coefficients of the Leslie ah hoc model are not independent, but connected. A comparison of the deduced relationship with experimental data is performed and an excellent agreement is obtained.     

Traveling waves in twisted nematic liquid crystal cells

We have described a novel reorientation mechanism in the form of the traveling waves, under influence of an external electric field, directed parallel to both glass plates, which occur in the twisted nematic cell (TNC). It is found that the slowest velocity of the traveling front is proportional to the field strength, and, approximately, in three times higher than the front velocity corresponding to the non-traveling solution. The value of the critical electric field Ecr$E_{\mathrm{cr}}$ which may excite the traveling waves in the TNC in π   times less than the value of the threshold electric field Eth$E_{\mathrm{th}}$ corresponding to the untwisted geometry.     

Simple Landau model of the smectic- A-isotropic phase transition

A simple Landau-type free energy function is presented to describe the smectic-A-isotropic phase transition. Varying the coupling between orientational and positional order parameters, a smectic-A-isotropic or a nematic-isotropic phase transition occurs. Within this model the smectic-A-isotropic phase transition is found to be always more strongly first order than the nematic-isotropic phase transition. The theoretical results are found to be in good agreement with all published experimental results. Received 27 June 2000     

Flow properties of the optically isotropic tetrahedratic phase

We discuss the coupling between flow and other hydrodynamic variables that can occur in the optically isotropic tetrahedratic phase (T_d) characterized by a third rank tensor order parameter T_ijk. We point out that an applied electric field or an applied temperature gradient will lead to flow. Reciprocally we predict that, for example, a shear flow applied to a tetrahedratic phase leads to an induced electric field and a temperature gradient. Similarities to recent experimental observations in the vicinity of the isotropic-B7 phase transitions in materials formed by banana-shaped molecules are discussed. Received 31 July 2001     

First order phase change detection using planar waveguide Bragg grating refractometer

Solid-to-liquid and gas-to-liquid phase changes in water and ordered-to-isotropic phase changes in a nematic liquid crystal are detected with an optical sensor. A planar Bragg grating defined purely by refractive index modulation is covered with a water or liquid crystal overcladding and the temperature is controlled to trigger phase changes. Measurement of the Bragg wavelength allows changes of effective refractive index to be detected and discontinuities in behaviour caused by phase transitions can be clearly identified.     

Induced ferrielectricity in antiferroelectric liquid crystals

The behaviour of the antiferroelectric SmC_A liquid crystal phase under applied electric field is discussed theoretically. The phase diagram involving the SmA, SmC_A and SmC _A ^* phases is worked out and shown to exhibit a Lifshitz critical point. The deformation of the bilayer structures induced by the field transforms the SmC_A phases into a ferrielectric phase whose specific configuration is described. Received: 23 October 1997 / Revised: 8 April 1998 / Accepted: 14 July 1998     

Stochastic processes in a nonlinear Kerr-like ordered liquid

A special case of the problem determined by laser-induced instabilities (at cw input) in wave interactions with inhomogeneous anisotropic media with high threshold nonlinearity under condition of energy competition and exchange between waves with orthogonal polarizations is examined.     

Surface anchoring energy and the Freedericksz transitions in ferronematics

This paper analyzes the influence of anchoring conditions on the two-field-induced Freedericksz transition in ferronematics. Using the Euler-Lagrange analytical method we find a correlation between the threshold intensities of the magnetic and the laser fields, the anchoring strength, the material and device parameters. The threshold fields in our case are smaller than those obtained in the rigid anchoring limit. We show that for finite anchoring energy, the saturation transition is possible, and find the correlation between the saturation fields, the anchoring strength, the material and cell parameters. Our results can be useful in designing magneto-optical devices using ferronematics.     

Thermal diffusivity measurements in a lyotropic discotic nematic phase

The present work is focused on thermal diffusivity (D)$(D)$ measurements, via Z  -scan experiment, in a discotic nematic phase of the lyotropic mixture comprised of potassium laurate, decanol and D₂O${\text{D}}_2\text{O}$. In this experiment, the nematic sample is translated through the focal region of a focused Gaussian laser beam. The experimental data are analyzed according to the thermal lens model and the ratio between the thermal diffusivities parallel (D_∥)$(D_{\parallel })$ and perpendicular (D_⊥)$(D_{\perp })$ to the director of the nematic sample has been found to be smaller than one. The results are compared to others obtained with different liquid crystals and explained by using a simple model where this ratio is correlated to the shape anisotropy of the micelles.     

Monte Carlo simulation of joint density of states in one-dimensional Lebwohl-Lasher model using Wang-Landau algorithm

Monte Carlo simulation using the Wang-Landau algorithm has been performed in an one-dimensional Lebwohl-Lasher model. Both one-dimensional and two-dimensional random walks have been carried out. The results are compared with the exact results which are available for this model.     

Phase transition in liquid crystal elastomers—A Monte Carlo study employing non-Boltzmann sampling

We investigate nematic-isotropic transition in liquid crystal elastomers employing a variant of Wang-Landau sampling. This technique facilitates calculation of the density of states from which other thermodynamic properties can be obtained. We consider a lattice model of a liquid crystal elastomer and a Hamiltonian which accounts for interactions among liquid crystalline units and interaction of local nematics with global strain. We investigate the effect of varying the strength of coupling between nematic and orientational degrees of freedom. When the local director is coupled strongly to the global strain, the transition is strongly first order. When the strength of the coupling decreases the transition becomes weakly first order. The transition temperature decreases when the coupling becomes weaker. We also report for the first time results on variation of free energy as a function of average energy at different temperatures and coupling constants.     

Non-equilibrium effects in spinodal decomposition of a binary system

A model for diffusion and phase separation which takes into account relaxation of the solute diffusion flux is developed. It is shown that the model predicts non-linearity in the amplification rate of decomposition, which is governed by the ratio between diffusion and correlation lengths. The predicted amplification rate is tested against experimental data on a binary phase-separated glass.