Crystallization kinetics of polydisperse colloidal hard spheres: Experimental evidence for local fractionation

We present the crystallization kinetics for two polydisperse hard-sphere particle stocks with differing particle size distributions. One of the latexes had a relatively symmetrical distribution, the other had a more polydisperse distribution, which was highly skewed to smaller sizes. The emerging Bragg reflections from the crystallizing samples were measured using a technique that provides improved statistical averaging over our previous methods. It was observed that, for the more polydisperse particles, the onset of nucleation was delayed by up to an order of magnitude in reduced time, and displayed qualitatively different growth behavior compared to the particles with the more symmetric size distribution. Based on these measurements and time lapse photographs, we propose a growth mechanism whereby crystallization occurs in conjunction with a local fractionation process near the crystal-fluid interface, which significantly alters the kinetics of crystallite nucleation and growth. This fractionation effect becomes more significant as polydispersity or skewness increases.