Phase separation in model colloidal liquids by Brownian dynamics simulations.

The technique of Brownian Dynamics simulation has been used to follow the evolution of model colloidal systems during phase separation in the liquid-vapour and solid-vapour regions of the phase diagram. Systems of monodisperse spherical particles interacting via LJ m:n type potentials were quenched in temperature from the one-phase region into the two phase region. Various structural and rheological properties were followed as the systems evolved, including the radial distribution functions, the small angle scattering peak of the structure factor, the interaction energy and the linear response rheology. The scaling behaviour of these quantities was found to be similar to that observed in light scattering experiments following the phase separation of colloidal systems. The aggregate structure could not be represented well by a single fractal dimension. Some evidence of fractal structure was found early in the phase separation, however the reversibility of the interactions allowed for a high degree of restructuring which led to a collapse of the initially tenuous structure into dense aggregates. The local structure was sensitive to the range of the interaction potential - as the potential became more short-ranged, increasing evidence of crystallisation of the denser phase was apparent from the form of g(f). Particles with 12:6 interactions formed structures displaying the rheological strength associated with an elastic gel. However restructuring was continual, resulting in a dense compact structure. The short-range 36:18 potential retained a tenuous gel-like structure and displayed an arrest of phase separation on long lengthscales. However, the particles did not have the interaction strength necessary to give significant rigidity to the system. This suggests that to form an arrested state with elastic gel-like rheology it would be necessary to have a more permanent form of interaction, in addition to the short-range reversible interactions used in this work