Showing papers by "Shri Singh published in 2010"

Thermodynamic model for the description of smectic C* and smectic A–smectic C* phase transition properties

Abstract: We construct a phenomenological model to describe the general features of chiral smectic C* (Sm C*) phase and Sm A–Sm C* phase transition properties of ferroelectric liquid crystals The free-energy density of the system is expanded in terms of three degrees of freedom – tensor orientational order Q ij , scalar smectic order Ψ and polarization vector P and couplings between these order parameters Since Q ij describes the tilt vector ξ, no term contains ξ The values of Landau coefficients are determined by comparing with the experimental data of helical pitch, tilt angle and spontaneous polarization and introducing an empirical relation for the temperature variation of the modulus S of orientational order parameter Q ij Taking these values we evaluate the tilt angle, spontaneous polarization and pitch of the Sm C* phase and the Sm A–Sm C* phase transition It is found that the theoretical results agree very well with the experimental data of four ferroic mesogens

Pressure-driven reentrant phenomena in liquid crystals: the role of inverse layer spacing

Abstract: The occurrence of pressure-driven reentrant phenomena observed in high-pressure experimental studies in some achiral mesogenic materials has been explained using a thermodynamic model based on Landau–de Gennes theory. In this approach, the free-energy is expanded in terms of nematic, smectic A order parameters and the couplings (cubic and biquadratic) between them. The basic theme here is that the ‘inverse layer spacing’, which mimics an order parameter, becomes coupled to the nematic and smectic A order parameters. Secondly, in addition to the order parameter couplings the N–SA metastable temperature (which appears due to elimination of inverse layer spacing from the free-energy density expression) becomes pressure dependent. The occurrence of a pressure-driven reentrant nematic phase is explained in terms of these three pressure-dependent parameters. They all show smooth but rapid variation at the critical pressure beyond which nematic reentrance appears.