Theoritical Study of Liquid Crystals in Porous Media

Abstract

LCs has some behaviour that are common in isotropic liquid and in solid crystal. The effect of surface on liquid crystal s behaviour like its transition and molecular arrangements influence a lot. newlineWhen LC imbedded in a any matrix, three types of effects are found which influence its behaviour. First the surface effect is found on the molecules that are very close to the surface newline19 newlinethat concerns the perturbation of liquid crystalline structure. Beyond this transition region, the bulk liquid crystalline structure is recovered with an orientation which is fixed by the surface, newlineA system has two types of anisotropy. It can be external (liquid or gas front propagating in a Hele Show cell with engraved plates) or intrinsic (gas propagating in a cell filled with liquid crystal, solidification). System with intrinsic anisotropy such as liquid crystals can show surface properties. [Buka et al. 1988]. Molecular interaction between a liquid crystal and ambient medium establish a definite orientation of the director n^. This deviation of director requires some energy. This altered orientation of liquid crystal by surfaces is called surface anchoring. Anisotropy is special characteristic found only in liquid crystals. Because of this they have different type of fluidity that make them separate from other matters. This property of liquid crystal determines the surface tension occurs between different mesophases. This determines the equilibrium shape of liquid crystals at coexistence temperatures. Different morphologies are obtained when the temperature is increased. As heat diffusion controls the molecular structure, anisotropy in thermal conductivity determines the change in molecular shape of liquid crystal in a particular direction. newlineExtensive tables are available listing practically all known mesogenic compounds and their transition temperatures. Two examples of rod like mesogens with phase transitions are illustrated below. Here the second transition is enantiotropic, that is, it takes place reversibly on heating and cooling, though the reversal to the solid phase is usually accompanied by supercooling. The one exception is cholesteryl monanoate which shows a montropic transition in which smectic A phase occurs on cooling. newline