CHANGE IN CRYSTALLIZATION MECHANISM AT THE GLASS-TRANSITION OF COLLOIDAL SPHERES

THE crystallization of 'hard' spheres, which interact only through repulsive forces on contact, is a purely entropic effect and provides a motel system for testing theories of freezing transitions in liquids generally1. Micrometre-sized colloidal particles can be made to approximate hard spheres by grafting of polymers to their surfaces1. Their slow dynamics make them attractive systems for the stud of crystallization kinetics2-4. As well as undergoing crystallization1,2, such particles also exhibit a glass transition at a well-defined volume fraction phi(g), which we have shown previously to be related to the cessation of large-scale diffusion on experimental timescales5,6. Here we show that the glass transition coincides with a change in the mechanism of crystallization. At volume fractions less than phi(g) isometric crystals are nucleated homogeneously throughout the sample, whereas above phi(g) crystals forming slowly in the metastable glass phase are highly asymmetric. Regular rocking of the suspension above the glass transition induces more rapid formation of plate-like crystals, indicating that their nucleation is shear-induced.