Pretransitional behaviour in nematogenic mixtures A molecular field theory

Abstract

The isotropic phase of nematogenic liquid crystals exhibits strong pretransitional behaviour which diverges rapidly as the transition to the nematic phase is approached. Similarly nematogenic mixtures also show pronounced pretransitional effects although these can be dramatically reversed on the formation of the biphasic regime when isotropic and nematic phases coexist. Here we develop a theory for such effects using approximations taken from previous molecular field theories of nematogenic mixtures. In particular we consider the composition dependence of the divergence temperature for binary nematogenic mixtures and its relation to those of the pure components. A variety of mixtures with particles of different shape are treated, namely rods and spheres, laths and spheres, rods and rods, as well as rods and laths. We also predict the temperature dependence of the field-induced order parameters, often used to monitor pretransitional behaviour, for certain mixtures in both the homogeneous isotropic phase and the subsequent biphasic regime. The predictions of the theory are found to be in good accord with those investigations of pretransitional behaviour which are available for nematogenic mixtures.