Out of equilibrium processes in oppositely charged colloidal suspensions

I will present some of the work I have recently done on nucleation and gel formation. In the first part of the talk I will present a numerical study of crystal nucleation in a binary suspension of oppositely charged colloids. Two different crystal structures compete in the thermodynamic conditions under study. We find that the crystal phase that nucleates is metastable and, more surprisingly, its nucleation free energy barrier is not the lowest one. This implies that, during nucleation, there is insufficient time for sub-critical nuclei to relax to their lowest free-energy structure. Such behavior is in direct contradiction with the common assumption that the phase that crystallizes most readily is the one with the lowest free-energy barrier for nucleation. The phenomenon that we describe could be relevant for crystallization experiments where competing solid structures are not connected by an easy transformation. In the second part of the talk I will present a study on gel formation for the same system. We analyze the mechanism by which a gel is formed starting from a homogeneous dilute suspension. Both the experiments and the simulations show that the mechanism is related to an arrested phase separation. We use Brownian Dynamics simulations in order to understand and rationalize the experimental observations. In particular, we focus on the relationship between interaction potential, equilibrium phase diagram and formation of out-of-equilibrium states.