Symmetry breaking in suspensions of anisotropic colloids : phase transitions, diffusion and effective interactions

Abstract

Colloids are particles with dimensions in between the nanometer and the micrometer suspended in a solvent. Similarly to the much smaller atoms and molecules, colloids appear in extremely different thermodynamic phases, such as gas, liquid, crystals and liquid crystals. Our interest focuses on the equilibrium thermodynamics of suspensions of anisotropic colloids, that is, colloids with a marked non-spherical shape. The non-spherical shape of these type of colloids allows them to develop novel macroscopic phases. These phases are identified with the peculiar type of ordering in space that colloids assume, or, in more technical terms, with their degree of spontaneous symmetry breaking. The aim of this thesis consists of describing and understanding various aspects of the symmetry-broken phases generated by anisotropic colloids: the conditions for phase transitions, the character of the equilibrium diffusion, and the features of different effective interactions. The bare interactions between colloids are modeled as hard anisotropic repulsions. Besides reproducing experimental conditions common in colloid science, hard-particle models allow to investigate the many-body behavior of a system in purely entropic terms, that is, independently of energy. Our analysis is based on the application of equilibrium classical statistical mechanics in the formulations of Density Functional Theory and Monte Carlo simulation. We study in Chapter 3 the effect of polydispersity on the liquid-crystal phase behavior of colloids having the shape of bricks (boardlike particles). We show that polydispersity appears to enhance the stability of the long-searched biaxial nematic liquid-crystal phase. In Chapter 4 we demonstrate that a similar effect can be achieved by introducing a non-adsorbing depletant into the suspension. We suggest that by manipulating the resulting depletion interaction one can tune the liquid-crystal phase behavior of the particles. In Chapter 5 we analyze the quality of a fundamental measure theory functional for the description of the freezing transition of model colloidal hard cubes and squares. In Chapter 6 we focus on the effect of the broken symmetry on the diffusion and time relaxation of rod-like colloids. Finally, in Chapter 7 we propose a simple theory for the description and prediction of the effective interactions between colloids induced by a solvent preferential adsorption