Anisotropic colloids: bulk phase behavior and equilibrium sedimentation

Abstract

This thesis focuses on the phase behavior of anisotropically shaped (i.e. non-spherical) colloids using computer simulations. Only hard-core interactions between the colloids are taken into account to investigate the effects of shape alone. The bulk phase behavior of three different shapes of colloids is studied, as well as the effect of gravity on the phase behavior of hard spheres and dumbbells. First we study the crystallization of hard spheres under gravity using simulations in the grand canonical ensemble, i.e. fixing the chemical potential. A surprisingly simple expression, based on local chemical equilibrium, for the chemical potential at which a layer of hard spheres crystallizes is shown to agree quantitatively with the simulation results. Then we study the bulk phase behavior of dumbbells, which consist of two overlapping spheres, focusing on the two crystals with inherent disorder, the plastic crystal and the aperiodic crystal. For very short dumbbells, as with spheres, the stable plastic crystal is of face centered cubic (FCC) type, while for slightly longer dumbbells the hexagonal close packed (HCP) plastic crystal is stable. For very long dumbbells, whose spheres are almost tangent, we show that the aperiodic crystal phase is stable, although its region of stability is quite small. The effect of gravity on a system of dumbbells is studied as well, showing the same phases as obtained in the absence of gravity. The simple expression that was shown to work well for hard spheres also describes the chemical potential of crystallization of dumbbells quite well, with the exception of the coexistence between the plastic and aligned crystals. The stacking behavior of a plastic crystal under gravity is investigated and shows a clear preference towards the HCP type, as expected from the bulk phase behavior of dumbbells with the same aspect ratio. Next, computer simulations on bowl-shaped model particles were compared to experiments on colloidal bowls in the form of collapsed shells. Both systems show long curved stacks upon compression from an isotropic fluid and the distribution of the lengths of these stacks was used to show the correspondence between the simulations and the experiments. Thin bowls, that are difficult to achieve using the current synthesis route, were shown to order spontaneously into a columnar phase in the simulations. The remarkably rich phase diagram of the bowls, as obtained from simulations, shows, aside from fluid and columnar phases, no less than four crystal phases. As a last shape we study discs with cusps or kinks in the outer surface, which are expected to model most colloidal and molecular discs more accurately than the conventionally used model, the cut sphere. We show that the cubatic phase, found for cut spheres, is not stable for our particles. Furthermore, we find an additional crystal phase, not found for cut spheres, that is very similar to a type of crystal that is commonly found for disk-like molecules. Additionally, the exact position of the phase boundaries is surprisingly sensitive to the subtle difference in shape between our cusp-free particle and the cut sphere.