The Cholesteric Blue Phases

Abstract

In 1888, the year commonly taken as the birthdate of liquid crystal research, F. Reinitzer wrote to O. Lehmann to describe the curious properties of cholesteryl benzoate, a “substance [which] has two melting points, if it can be expressed in such a manner.” Throughout most of the 33°C interval between these two “melting points” this material is in the birefringent fluid state now known as the cholesteric liquid crystal. Today it is common to find compounds showing a whole cascade of liquid crystalline mesophases as the temperature is increased, but it is not customary to refer to any of the phase changes between them as “melting points” except for the lowest temperature transition where the crystalline lattice dissolves. In recent years, however, it has been discovered that many cholesteric liquid crystals, includin g cholesteryl benzoate, do something very strange in a temperature interval of only a degree or so just before they yield up their last bit of liquid crystalline order: they form complex structures having the symmetries of cubic lattices — they “freeze”! – and then “melt” at a higher temperature into either the ordinary amorphous liquid or else into a new kind of amorphous liquid which in turn undergoes a sharp transition into the ordinary amorphous liquid at still higher temperature.