Effect of an adsorbing surface on the phase behavior of a confined semiflexible liquid crystal

Abstract

Results of a study of the nematic to isotropic transition of a liquid crystal near an adsorbing surface in a thin film using Monte Carlo simulation are presented. The order parameter profile, heat capacity and overall order parameter are calculated to characterize the system as it undergoes the transition. We show that the strength of the adsorption energy has a large influence on the phase behavior of the system. At low adsorption energies a boundary layer is present which undergoes a transition from a nematic to a partially ordered phase at the reduced temperature τ1 and then undergoes a transition to an isotropic orientation at the reduced temperature τ2 which is separate from the bulk. The bulk undergoes an order–disorder transition at a temperature τ3 between the two orientation transitions τ1 and τ2 of the boundary layer. For strong adsorption energies, the boundary layer increases to the thickness of the sample. In this regime, the complete system undergoes the nematic–isotropic transition simultaneously and the temperature of this transition increases linearly with the adsorption energy. The case of intermediate adsorption energies corresponds to a crossover between the above two limiting situations. Here, the boundary layer is more disordered than the bulk and, yet, the entire sample undergoes the nematic–isotropic transition simultaneously. The observed phase behavior is explained in terms of the influence of the boundary layer on the order of the system. The present situations suggest the existence of a critical adsorption energy above which there is only one transition but below which several transitions occur due to the intervention of the boundary layer.