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.