Physics of inhomogeneous nematic liquid crystals : colloidal dispersions and multiple scattering of light

Abstract

The habilitation thesis deals with two interesting aspects of nematic liquid crystals with an inhomogeneous orientational order induced either by dispersed particles or by thermal director fluctuations. In the first part, the phenomenological description of the nematic phase and its topological defects are reviewed. The second part addresses the physics of nematic colloidal dispersions as a novel challenging type of soft matter. We first investigate the nematic environment of one particle with homeotropic boundary condition. Three possible structures are identified and discussed in detail; the dipole, the Saturn-ring and the surface-ring configuration. Secondly, we treat dipolar and quadrupolar two-particle interactions with the help of a phenomenological theory. Thirdly, we calculate the anisotropic Stokes drag of a particle in a nematic environment which determines its Brownian motion. We then turn our interest towards colloidal dispersions in complex geometries where we identify the dipolar configuration and study its formation. Finally, we demonstrate that surface-induced nematic order above the nematic-isotropic phase transition results in a strongly attractive but short-range two-particle interaction. Its strength can be controlled by temperature and thereby induce flocculation in an otherwise stabilized dispersion. In the third part we study multiple scattering of light from thermal fluctuations of the director. We use this scattering mechanism to test our generalized theory for the diffuse transport of light and its temporal correlations in random anisotropic media. Diffusing light constitutes a successful regime for accessing multiply scattered light. In diffusing-wave spectroscopy it is used to monitor the dynamics of turbid systems. We first provide a review of all the fascinating facets of multiply scattered light, and we introduce the basic theory of diffuse light transport in isotropic systems.