What happens at freezing. A perspective from suspension of hard sphere-like particles

Just how a fluid freezes into a crystalline solid remains among the most basic curiosities in the science of condensed matter. Thermodynamics tell us that freezing, or a first order phase transition generally, occurs when there is a singularity in the free energy. Either side of the singularity the material is in either the equilibrated fluid phase or equilibrated crystal phase - each being independent of the other, both structurally and dynamically. A careful analysis of the dynamics of particles in suspension, measured by laser light scattering, indicates this perspective offered by thermodynamics may be incomplete. Previous work has established that certain suspensions of near-micrometer sized particles show a transition, from a disordered, colloidal melt phase to an ordered, colloidal crystal phase, that mimics the freezing-melting transition of the ideal system of hard spheres. Being hard spheres, whose motions in the suspending liquid are overdamped, allows the collective consequences of packing constraints to be isolated. In these we identify a component on the fluid side of the transition that is anisotropic and, as such, is a characteristic of the crystal phase. On extrapolation these heterophase fluctuations attain criticality at a volume fraction that coincides with the observed 'freezing point’. Traversal of this point is accompanied by resistance to compression of the most populated spatial modes.