Entropically Driven Surface Phase Separation in Binary Colloidal Mixtures

We investigate the phase diagram of mixtures of charge stabilized polystyrene spheres of two different sizes in the hard-sphere limit. We observe bulk phase separation into two disordered phases and find a new ordered phase located on the cell walls. The ordered phase occurs at volume fractions as low as 0.2, much less than the value of 0.49 for hard-sphere freezing in monodisperse suspensions, and is qualitatively explained with simple geometric arguments.