Experimental evidence for two-step nucleation in colloidal crystallization.

We investigated the freezing of colloidal spheres in two dimensions with single-particle resolution. Using micron-size, charge-stabilized polystyrene spheres with a temperature-dependent depletion attraction induced by surfactant micelles, we supercooled an initially amorphous (gaslike) system. Particle motions were monitored as crystallization proceeded. At low concentrations, freezing occurred in a single step in a manner consistent with classical nucleation theory. In other samples two-step nucleation was found, in which amorphous clusters grew to approximately 30 particles, then rapidly crystallized. Measured free energies show the role of metastable gas-liquid coexistence, which also enhanced the rate of nucleation following deeper quenches.