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Non-equilibrium dynamics of solidification: new perspectives from colloidal hard spheres

Freezing of a liquid into a crystalline solid when cooled is a familiar occurrence. In the process, some particles become caged: their movement hindered by the close proximity of neighbouring particles. The nature of the cooperative processes that ensue among the caged particles, how those processes facilitate or impede the separation of the crystal from the super-cooled fluid, are far from understood. We address this shortcoming primarily with the results of experiments with suspensions of hard sphere particles. These show a first order fluid to crystal transition. The delay in its onset then exposes, in the density quenched super-packed suspension, a quasi-stationary dynamical heterogeneity in which the solid component comprises an amorphous assembly of caged particles. That solid’s stability is maintained by non-propagating transverse phonons, whose bandwidth is proportional to the fraction of particles that are caged. Time-scale invariance of the phonons distinguishes the amorphous solid from the crystalline solid. We find that the broadening of the bandwidth of those phonons reaches a threshold where propagating phonons – elastic
modes – set in. Their quantisation amounts to the formation of the crystal phase.