Smectic Liquid Crystals

Abstract

This chapter is devoted to the smectic mesophases that are characterized by both the orientational order of the long-molecular axes and a reduced positional order. As discussed in chapter 1 (sec. 1.4), smectics have stratified structures with a well defined interlayer spacing; within each stratification a variety of molecular arrangements are possible. With the established layer structure, two kinds of phases, orthogonal and tilted, can be distinguished. In the orthogonal phases the director is perpendicular to the layers or equivalently parallel to the direction of the density wave. Tilted smectic phases are characterized by an angle between the director and normal to the smectic planes. The term "Smectic" was coined by G. Friedel to designate certain mesophases observed in soaps and lipid-water mixtures. He recognized only one kind of smectic -now known as smectic A (SA). A number of smectic phases has now been identified giving rise to different macroscopic textures (see chapter 11) which are recognized by optical observation. A natural classification of these phases should be made on the basis of their symmetry properties. However, their notation is according to the order of discovery of the different phases and bears no relation with the symmetry and molecular packing.