Surface and electric field effects on phase transitions in smectogenic Systems involving the Smectic-Cp Phase

Abstract

This thesis investigates surface and finite size effects on the phase transitions of free-standing smectic-C films and electric field effects in the bulk. Initially, a review is given of existing and widely used microscopic models for the description of phase transitions in liquid-crystalline systems presenting nematic and smectic-A phases. We then review the model developed by Govind and Madhusudana, describing the smectic-C–smectic-A phase transition. Motivated by their approach, we propose a new theoretical model to describe phase transitions in systems presenting the smectic-C phase. Using a single-particle mean-field potential, we show that the phase diagram of liquid-crystalline compounds with a small transverse dipole can be reasonably described when the tilt-induced contraction of smectic layers is considered and with a reduced number of model parameters. Aiming to extend the proposed model for freely suspended films, McMillan-Mirantsev’s model for free-standing smectic-A films is reviewed. The model introduced by Mirantsev shows that surface anchoring stabilises the smectic hase for temperatures above the bulk transition temperature. Considering a tilt angle profile and a discrete version of the single-particle mean-field potential, the interplay of surface anchoring and finite size effects affects the behaviour of the order parameters in free-standing smectic-C films. The transition temperature is shown to depend on surface ordering and film thickness. The orientational order imposed by surface anchoring is shown to stabilise the tilt order parameter in the film surface above the bulk smectic-C–smectic-A transition temperature. Surface layers therefore remain in the smectic-C phase, while the central layers undergo a smectic-C–smectic-A phase transition. The effect of an external electric field is considered in smectic-C bulk samples, inducing a reorientation of the irector vector and thus enabling the suppression of the isotropic phase and the appearance of residual and field-induced nematic phases.