Surface phase transitions in liquid crystals

Braun, Frank Nicholas (1996) Surface phase transitions in liquid crystals. University of Southampton, Doctoral Thesis.

Abstract

We discuss general features of the relation between wetting properties and temperature dependence of the free surface tension close to the clearing point. Different wetting properties are expected to yield a varied assortment of surface tension characteristics. In order to supplement and extend these arguments, we have analysed the surface tension using a microscopic density functional model. In this approach anisotropic volume exclusion effects are neglected, isolating instead the effect on the surface tension of the orientationally coupled Lennard-Jones potential. Wetting properties are sensitive to the different spherical harmonic components of this potential, and thus the model exhibits a rich array of surface tension behaviours, of which we give an overview. In particular, our calculations reveal a pre-transitional surface tension minimum which is seen in many experimental studies. This is related to anomalously high sub-surface order.

We next generalise the density functional model to consider oblique anchoring at the free surface. This is achieved by including a term with the symmetry of the electrostatic quadrupole in the model molecular interaction potential. As the bulk triple point of the liquid crystal is approached, the model exhibits a well-known second order oblique to homeotropic anchoring transition observed experimentally. The result constitutes the first thermodynamically consistent demonstration of this phenomenon by a microscopic theory. We plot the relevant anchoring phase diagrams, and compare them with constrained treatments of the density functional. By means of this comparison we assess to what extent the commonly adopted strategy of decoupling interfacial structure from anchoring is justified.

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