1. Introduction

Abstract

An exceptional variety of phase transitions is characteristic of a liquid crystalline state¹. Quite often one encounters several different liquid crystalline phases appearing in an interval of only a few degrees (Figure 1). These transitions are usually the second order or weakly first order. Therefore they are accompanied by critical phenomena. Here “critical phenomena” is meant to denote those phenomena which are peculiar to the vicinity of second order transitions or critical points, i.e, they exhibit a huge increase of susceptibility, an anomaly in the specific heat, large fluctuations of order parameter, etc. Critical phenomena in liquid crystals have specific features due to the variety of the symmetries of the different phases and coupling of the different order parameters. These features complicate theoretical considerations and restrict methods which are well proved in other cases (e.g. renormalization group method). Lack of a strict theory often leads the experimentalists to interpret the results of their investigations in terms of mean-field approximation only. From the experimental point, of view, the liquid crystal near a phase transition is a more “capncious” object than fluids in the vicinity of their critical points. All defects of solids are peculiar to liquid crystals: dislocations, quenched (“frozen”) impurities, and the practical inability to intermix the viscous phases.