DIRECT OBSERVATION OF OSCILLATORY-SHEAR-INDUCED ORDER IN COLLOIDAL SUSPENSIONS

We use optical microscopy to directly study ordering in concentrated c olloidal suspensions of model hard spheres subjected to oscillatory sh ear in a parallel. plate geometry. At high strain amplitude the partic les order quickly into hexagonal planes in the plane of shear, oriente d so that a close-packed direction is parallel to the velocity axis, c orresponding to previous scattering observations in other geometries. At low strain a polycrystalline structure forms, consisting of hexagon ally ordered regions now with a distribution of different orientations , peaked around an orientation where a close-packed direction is now p erpendicular to the velocity axis. Such polycrystallinity explains the evidence of disorder previously noted in scattering patterns. The ord ered regions grow until ''grains'' fill the sample. Various examples o f local disorder are observed,in the shear-ordered structures, includi ng vacancies, dislocations, and stacking faults. The observed.motion o f the planes during the cycle is complicated, but there is some eviden ce of a ''zigzag'' motion of planes at high strain, as has been propos ed in previous studies.