Theories of Nematic Order

The paper starts with a description of the molecular alignment that characterizes nematic liquid crystals, and a brisk review of the classic mean field theories of this alignment and of recent attempts to improve on the mean field approach by introducing the direct correlation function. It is argued that such theories are unsatisfactory on two grounds: (a) because they fail to recognize that correlations of orientation between adjacent molecules are of very much longer range than the intermolecular potential -- they fall off, indeed, only as fast as 1/R -- and (b) because they fail to allow for director fluctuations. The author's continuum theory, which attributes entirely to director fluctuations the fact that molecules in nematics are not perfectly aligned, is free from these particular objections, and it seems to give the most complete description currently available, especially at low temperatures, for the behaviour of simple model nematics that have been studied by computer simulation. Its failure to match completely the behaviour of real nematics such as 5CB may be due to their polar character.