Colloidal glasses under shear strain

We have studied the effects of oscillatory shear strain on the structure of a colloidal hard-sphere glass. By light scattering, we measure the kinetic development of ordered structures induced by shearing between parallel plates. Diffusing wave spectroscopy ``echo'' experiments show that for peak to peak oscillatory strain below around 30% the colloidal glass strains approximately reversibly. At higher strains a partly ordered structure develops, the kinetics of the ordering being strongly dependent on strain. Kinetic measurements demonstrate an ``induction'' time for crystallization with a divergentlike behavior as strain decreases toward ~25--30%. We compare the behavior of glassy samples with that of samples at lower volume fraction, where in equilibrium without shear the sample is fully (poly)crystalline. At the lower volume fraction, irreversible yielding is observed at lower strains. There appears some tendency for very high volume fraction glass samples to ``fracture.'' Similar evidence of fracture is not observed at the lower volume fractions.