Transições de fase e efeitos da adição de nanopartículas em sistemas líquido-cristalinos

Abstract

In the present work, we study the effects of external fields and surfaces on liquid-crystalline systems. Initially, we investigate the effects caused by the addition of ferroelectric nanoparticles in a nematic liquid crystal. Using the technique of molecular dynamics simulation, we estimate the density of liquid crystal molecules, the orientational order parameter, and the polar and azimuthal angle profiles as a function of the distance to the center of the immersed nanoparticle for different temperatures of the system. We observe that the presence of ferroelectric nanoparticles enhance the nematic order of the liquid-crystalline medium changing many properties above the transition temperature Tn — Iso. In the second moment, we determine the interaction mediated by elastic deformations between colloidal nanoparticles adsorbed on the surface of free-standing smectic films. In these films, the elastic-mediated force between the adsorbed particles have a long-range character, which contrasts with the character of short-range in films on a solid substrate. The study of phase transitions and critical phenomena in free-standing smectic films has been a subject of many theoretical and experimental works. The McMillan-Miranstsev mean-field approach was used to show that the additional orientational order imposed by the surface anchoring stabilizes the surface-induced smectic siSmA and nematic N phases, leading to he collapse of the tricritical point and to the emergence of a critical end point. Usually strongly anchored free-standing smectic exhibit a stepwise reduction in the thickness when the temperature is raised above the smectic-isotropic bulk transition temperature. In this study, we demonstrate that a layer trinning transition, induced by and external field, can occur in smectic films with negative dielectric anisotropy even below the bulk transition temperature. Using an extended McMillian's model, we show that when the field increases above the bulk transition field, the film thickness reduction is well described by a power law.