OSCILLATORY-SHEAR-INDUCED ORDER IN NONAQUEOUS DISPERSIONS OF CHARGED COLLOIDAL SPHERES

A light scattering study is presented of the nonequilibrium microstruc ture of nonaqueous dispersions of repulsive charged colloidal silica s pheres subjected to an oscillatory shear how. Two ordered structures a re induced upon increasing the oscillation amplitude: alternating face -centred-cubic (fcc) twins and registered randomly stacked sliding lay ers. The transition between these structures shifts to lower amplitude s as particle volume fraction is increased. This is consistent with pr edictions of a simple strained fcc-lattice packing model previously de veloped for hard spheres. There are, however, quantitative differences due to the softness of the interparticle interactions of our system. In addition, we find that the fcc twins appear much more slowly than t he sliding layers and that the former occurs mainly due to shear orien tation of existing crystallites. We also observe that the formation of the sliding layers depends on the amplitude as well as the frequency of oscillation.