Morphology of crystals made of hard spheres

The growth and morphology of crystals of hard colloidal spheres is monitored by a special photographic technique. Crystals are observed to grow in size and increase in number density, initially. This is followed by a decrease in size and an increase to saturation in the number density. During the growth stage the crystals evidence irregular rectangular shapes, and an internal substructure is evident within the larger crystals. The observed size decrease suggests a breakup of crystals to a length scale of the internal structure. A simple model for surface roughening, which has been used to classify crystal morphology, is. examined for hard sphere crystals. Hard sphere crystals should be classified as ''rough'' and subject to growth instabilities. The length scale for simple planar and spherical instabilities is examined for hard sphere crystals and is found to be smaller than the observed length scales.