THE MICROSCOPIC DYNAMICS OF FREEZING IN SUPERCOOLED COLLOIDAL FLUIDS

Using time-resolved digital video microscopy, we have tracked the reem ergence of order in charge-stabilized colloidal crystals which have be en shear melted into isotropic fluids. Crystallization is heterogeneou sly nucleated by the smooth walls of the sample container. This proces s is analogous to the solidification of conventional materials during casting or liquid phase epitaxy. The nonequilibrium freezing transitio n proceeds through the gradual formation of a layered fluid near the r epulsive wall, subsequent evolution of local order within the first fl uid layer, and ultimately rapid crystallization. After nucleation and initial growth, crystallites are observed to fracture, perhaps due to shear stresses imposed by neighboring crystallites. Microscopic measur ements of the nonequilibrium self-diffusion coefficient are consistent with the dynamical freezing criterion recently proposed for systems i n equilibrium by Lowen et al. [Phys. Rev. Lett. 70, 1557 (1993)].