(198e) Contributions of Dynamical Heterogeneities to Non-Linear Rheology of Confined Supercooled Colloidal Liquids Under Oscillatory Shear

We have recently investigated the structural dynamics of confined amorphous colloidal thin films under oscillatory shear using a home-designed micron-gap rheometer interfaced with a confocal microscope. We focus on model ?hard-sphere? poly-(methyl methacrylate) (PMMA) particles of 1.2 um in diameter suspended in the density and refractive index matched nonpolar solvent at particle volume fraction of 0.43-0.57. We simultaneously visualize the dynamical response of confined PMMA particles between two solid surfaces at narrow gap spacing of 5-80 µm to applied shear deformation and measure the viscoelasticity of PMMA colloidal thin films. For confined PMMA colloids under shear where an applied deformation is sufficiently large to induce non-linear rheological responses, we find significant non-universality in their plastic behavior. Non-affine motion, determined by subtracting the globally uniform strain from the bare particle coordinates, reveals that particles move as cooperatively rearranging groups that exhibit strong anisotropy and coexist with long lived immobile regions at narrow gaps in contrast to the isotropy observed in the bulk; the length scales of these groups typically approach the order of spatial confinement dimension. We also examine the role of the film-thickness dependent anisotropy on shear thickening observed in the limit of large strain amplitudes.