THE MICROSCOPIC DYNAMICS OF FREEZING IN SUPERCOOLED COLLOIDAL FLUIDS

Using time-resolved digital video microscopy, we have tracked the reemergence of order in charge-stabilized colloidal crystals which have been shear melted into isotropic fluids. Crystallization is heterogeneously nucleated by the smooth walls of the sample container. This process is analogous to the solidification of conventional materials during casting or liquid phase epitaxy. The nonequilibrium freezing transition proceeds through the gradual formation of a layered fluid near the repulsive wall, subsequent evolution of local order within the first fluid 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 measurements of the nonequilibrium self-diffusion coefficient are consistent with the dynamical freezing criterion recently proposed for systems in equilibrium by Lowen et al. [Phys. Rev. Lett. 70, 1557 (1993)].