Anisotropic Heterogeneous Dynamics in Confined Colloidal Liquids under Oscillatory Shear

We have investigated the dynamics of confined amorphous ``hard-sphere'' colloidal suspensions under oscillatory shear using a home-designed micron-gap rheometer interfaced with a confocal microscope. We have focused on model hard-sphere colloidal suspensions of micron-sized poly-(methyl methacrylate) (PMMA) particles suspended in density and refractive index matched nonpolar solvents at particle volume fractions, $\phi $ = 0.40 and 0.43. We simultaneously visualize the dynamical response of confined PMMA particles between two solid surfaces at narrow gap spacing of 10-28 particle layers to applied shear deformation and measure their viscoelasticity. Above a threshold strain of $\sim $6{\%} where an applied deformation is sufficient to induce plastic behavior, we find that structural rearrangements are highly anisotropic. Non-affine motion, determined by subtracting the globally uniform strain from the bare particle coordinates, reveals that particles move as cooperatively rearranging groups with a preferred orientation along the flow direction. Metrics which probe cooperative dynamics all reveal a strong amplitude, thickness, and directional dependence on the characteristic sizes of the cooperatively rearranging regions.