Efeitos de superfície nas propriedades térmicas e dinâmicas de filmes de cristais líquidos esméticos. / Surface effects on the thermal and dynamical proprieties of smectic liquid crystal filmes.

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

The thermal and dynamical properties of free-standing films are strongly dependent on the boundary conditions and the presence of an external field. Using distinct theoretical models and the dynamic light scattering experimental setup, we study the surface and external field effects on the relaxation dynamics, fluctuation-induced interaction energy, and phase transitions of smectic films. Using an extended McMillan model, we study as a homeotropic anchoring stabilizes the smetic order above the bulk transition temperature of the system. In particular, we determine as the transition temperature depends on the surface ordering and film thickness. We identify a characteristic anchoring for which the transition temperature does not depend on the film thickness. For strong surface ordering, we observe that the transition temperature presents a power law scaling behavior with the film thickness, in agreement with a series of existing experimental results in the literature. Within a Gaussian functional approximation, we also investigate how the fluctuationinduced interaction force depends on the nematic and smectic order parameter profiles. Close to nematic-smectic phase transition, our results indicate that the thermal Casimir force has a significant enhancement which reinforces the predominance of the fluctuationinduced interaction as compared to standard van der Waals interaction in thin smectic films. In temperatures where the smectic phase is well established, we investigate how the interplay of the surface order and the external field induced order affect the Casimirlike force. For asymmetrically anchored films, the fluctuation-induced interaction energy obeys a law of corresponding states and can change its nature from attractive to repulsive through variations of an external field. We discuss the possible relevance of this field effect in smectic wetting transitions. With concern to the dynamical properties, we investigate how surface operators modify the relaxation dynamics of hexatic-B liquid crystal films. In particular, we study how a surface ordering field modifies the damping rate of the orientational modes in the system. In a purely diffusive regime, we demonstrate that a surface tilted order drives the slowest relaxation mode from hydrodynamical to non-hydrodynamical character. In the viscous regime, the hydrodynamical character of the slowest relaxation mode persists even for a surface tilted order. However, the normal modes develop an oscillatory-exponential relaxation. Finally, we use the photon correlation spectroscopy technique to investigate the dynamic properties of free-standing films close to smectic-A - crystal-B transition. We observe that the experimental data indicate the existence of a non-hydrodynamic mode and measure the temperature dependence of the damping rate. We considered a simplified model that incorporates the gradient velocity field effects which reproduces qualitatively the experimental measurements.

ASSUNTO(S)

long-range forces transição de fase fisica phase tramitions dinâmica de relaxação cristais líquidos liquid crystal teoria do espalhamento light scattering relaxation dynamics força de longo alcance

ACESSO AO ARTIGO

Documentos Relacionados