Subir Kumar Roy

Birefringence and Order Parameter of Some Alkyl and Alkoxycyanobiphenyl Liquid Crystals

Abstract The refractive indices and densities of six liquid crystals—two alkylcyanobiphenyls, three alkoxycyanobiphenyls and one mixture of two alkylcyanobiphenyls have been reported as a function of temperature. The effective polarizabilities αeand αo in the nematic phase calculated using the methods due to Neugebauer, Vuks and Saupe and Maier, are found to be appreciably different, though the order parameter S evaluated with those values of (αe - αo) are in good agreement. The preference of one method over the other two is discussed.

Refractive Indices and Dielectric Studies of Three Phenylcyclohexane Liquid Crystals in the Nematic Phase

The refractive indices ne, no and densities and the dielectric primittivities e∥ and e⊥ of three P-cyano-P-alkylcyclohexanes (PHC-3, PCH-5, PCH-7) have been measured at different temperatures. The ...

Ferroelectric liquid crystal cell versus dye doped ferroelectric liquid crystal cells: A comparison of dielectric properties

We report the dielectric dispersion and ferroelectric response of a ferroelectric liquid crystal (FLC) mixture after doping with anthraquinone dye at two different concentrations (0.5% and 2.5% w/w). Dielectric and polarization switching properties of the FLC mixture and that of the dye-doped FLC mixtures are investigated as a function of temperature. It is observed that the dielectric data in the low concentration dye-doped mixture are comparable to that of the undoped FLC compound. On the other hand the real part of dielectric permittivity is significantly reduced when the concentration of the dye is increased to 2.5%. Temperature dependence of spontaneous polarization and response time is measured and that supports the dielectric results. We also evaluated the rotational viscosity and activation energy. The SmC*-SmA* transition temperature is decreased by the addition of dye and that shift with temperature is discussed with the help of the Landau theory.

Effect of UV Curable Polymer on The Dielectric & Electro-Optic Properties of Ferroelectric Liquid Crystal

Results reported on the dielectric properties, spontaneous polarization (Ps) and switching time of a FLC material and a mixture of the FLC material after having doped with a small quantity of optical adhesive polymer. The mixture has been prepared by dissolving NOA65 prepolymer and the FLC compound in chloroform. A homogenous mixture of FLC and polymer formed after chloroform is being evaporated. Dielectric and Ps measurements of FLC-Polymer (PSFLC) mixture have been performed in a 10 w m ITO coated cell after the cell being irradiated with UV radiation for Photo-polymerization in situ. It has been observed that the spontaneous polarization of PSFLC mixture is unexpectedly reduced in polymer network formed as a result of photo polymerization. Similarly the Goldstone mode dielectric strength is also significantly reduced and switching time has been increased. The Soft mode dielectric strength however remains unchanged. The reduction of spontaneous polarization and Goldstone mode dielectric strength of the PSFLC mixture is not the accordance with the mixing ratio. The results have been discussed.

Bias dependent dielectric relaxation dynamics of electrically tuned large-scale aligned zinc oxide nanorods in nematic liquid crystal host

It was observed that mixed ZnO nanorods and twisted nematic liquid crystals (LCs) show highly ordered molecular system. The observed molecular relaxation arose due to a reorientation of long molecular axis of the order of 400 kHz for pure ZLI-1636, while such relaxation was shifted to a lower frequency for mixture. By using the proposed equations, f=fDB−α exp[−EV/(B−BC)] and σ=σDB−sα exp[−sEV/(B−BC)], the obtained equivalent activation energy was increased remarkably by the introduction of ZnO nanorods in pure LC. A strong long-range interaction is established in 0.01% ZnO nanorods mixed LC system.

Effect of spontaneous polarization on smectic C*-smectic A* phase transition temperature and the thickness dependence of the spontaneous polarization of ferroelectric liquid crystal

The influence of spontaneous polarization on the smectic C*-smectic A* phase transition was studied theoretically by means of Landau theory. calculations that the transition temperature from a non-chiral to chiral smectic C phase is shifted due to the effect of bilinear and biquadratic couplings. A qualitative relationship has also been derived between spontaneous polarization and cell thickness which supports the experimental observations of the increase of spontaneous polarization with the increase of cell thickness for surface-stabilized ferroelectric liquid crystals. It was observed from the theoretical

Dielectric Properties of a Ferroelectric Liquid Crystal Mixture Under Bias Electric Field

The dielectric properties of a ferroelectric liquid crystal mixture of low spontaneous polarization have been studied here as a function of D.C. bias voltage, frequency and cell thickness. A small sharp peak of dielectric constant is found to become prominent at the chiral smectic C-smectic A phase transition under the condition in which Goldstone mode is suppressed. The relaxation frequency, dielectric strength and the rotational viscosity of the Goldstone mode have been determined under D.C. field.

Soft mode dielectric relaxation under the influence of bias electric field of a ferroelectric liquid crystal mixture

Abstract Temperature dependent soft mode part of the complex dielectric constant has been measured as a function of bias electric field both in the smectic C* and smectic A* phases for a ferroelectrc liquid crystal (FLC) mixture. From the experimental data, soft mode relaxation frequency and dielectric strength have been determined both in smectic C* and smectic A* phases. In smectic A* phase, relaxation frequency and inverse of dielectric stmngth of the FLC mixture does not follow the Curie-Weiss law. The critical exponent for the studied FLC mixture is 1.43. Dielectric results shows a dependence of soft mode dielectric strength and relaxation frequency with bias field near the tmudion temperature in smectic A* phase to an extent of 1°C above the transition tanperature and above 1°C bias field does not have any effect on relaxation frequency and dielectric strength It is observed that relexation frequency increases with increasing bias field while the compounding dielectric strength decreases.

Influence of Piezoelectric Bilinear and Biquadratic Coupling in Smectic C* Phase of Ferroelectric Liquid Crystal

A theoretical relation has been developed from the Landau free energy expansion between SmC–SmA phase transition temperature of chiral smectic compound and that of its nonchiral analogue. The bilinear and biquadratic coupling constant and the quadrupolar order parameter have been determined from the extended microscopic model in the chiral smectic-C (Sm-C*) phase of four ferroelectric liquid crystal (FLC) materials and results have been discussed.

Effect of cadmium sulfide nanorod content on Freedericksz threshold voltage, splay and bend elastic constants in liquid-crystal nanocomposites

This report describes how doping liquid crystals (LC) with rod-like hexagonal semiconductor nanoprisms alters the dielectric and elastic properties of the composites as compared with a pristine nematic liquid crystal (NLC). Cadmium sulfide nanorods were synthesized via the solvothermal process and blended with a NLC. Nanorods were highly miscible with NLC and produced a topological defect-free texture up to a certain limit. A good dark state was achieved during the homeotropic configuration of the cell within that limit. Appreciable changes in splay and bend elastic constants of the LCs were observed after blending with nanorods. Long-range order was established in the hybrid system, and consequently the anisotropy was increased. The threshold voltage decreased dramatically by ?31%. Dielectric study revealed a high-frequency mode, which might be due to anchoring of the LC with nanorods.