Oscillatory-shear-induced order in nonaqueous dispersions of charged colloidal spheres

A light scattering study is presented of the nonequilibrium microstructure of nonaqueous dispersions of repulsive charged colloidal silica spheres subjected to an oscillatory shear flow. Two ordered structures are induced upon increasing the oscillation amplitude: alternating face-centred-cubic (fcc) twins and registered randomly stacked sliding layers. The transition between these structures shifts to lower amplitudes as particle volume fraction is increased. This is consistent with predictions of a simple strained fcc-lattice packing model previously developed 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 the sliding layers and that the former occurs mainly due to shear orientation of existing crystallites. We also observe that the formation of the sliding layers depends on the amplitude as well as the frequency of oscillation.