Direct observation of oscillatory-shear-induced order in colloidal suspensions

We use optical microscopy to directly study ordering in concentrated colloidal suspensions of model hard spheres subjected to oscillatory shear in a parallel plate geometry. At high strain amplitude the particles order quickly into hexagonal planes in the plane of shear, oriented so that a close-packed direction is parallel to the velocity axis, corresponding to previous scattering observations in other geometries. At low strain a polycrystalline structure forms, consisting of hexagonally ordered regions now with a distribution of different orientations, peaked around an orientation where a close-packed direction is now perpendicular to the velocity axis. Such polycrystallinity explains the evidence of disorder previously noted in scattering patterns. The ordered regions grow until ``grains'' fill the sample. Various examples of local disorder are observed in the shear-ordered structures, including vacancies, dislocations, and stacking faults. The observed motion of the planes during the cycle is complicated, but there is some evidence of a ``zigzag'' motion of planes at high strain, as has been proposed in previous studies.