Structure and kinetics in the freezing of nearly hard spheres

We consider homogeneous crystallisation rates in confocal microscopy experiments on colloidal nearly hard spheres at the single particle level. These we compare with Brownian dynamics simulations by carefully modelling the softness in the colloid interactions with a Yukawa potential, which takes account of the electrostatic charges present in the experimental system. Both structure and dynamics of the colloidal fluid are very well matched between experiment and simulation, so we have confidence that the system simulated is close to that in the experiment. In the regimes we can access, we find reasonable agreement in crystallisation rates between experiment and simulations, noting that the larger system size in experiments enables the formation of critical nuclei and hence crystallisation at lower supersaturations than in the simulations. We further examine the metastable fluid with a novel structural analysis, the topological cluster classification. We find that at densities where the hard sphere fluid becomes metastable, the dominant structure is a cluster of m = 10 particles with five-fold symmetry. Analysing histories of the local environment of single particles, we find fluctuations into crystalline configurations in the metastable fluid, and that the crystalline state a very often preceeded by a transition region of frequent hopping between crystal-like environments and other (m ≠ 10) structures.