Oscillatory shear-induced 3D crystalline order in colloidal hard-sphere fluids

Abstract

The non-equilibrium phase behavior of a colloidal hard-sphere fluid under oscillatory shear was investigated in real-space with experiments on poly(methyl methacrylate) (PMMA) colloidal suspensions and Brownian Dynamics computer simulations. All the samples in both experiments and the simulation are below the coexistence density of hard-sphere freezing, so the shear-induced crystals are out-of-equilibrium and melt after cessation of the shear. The physics is therefore fundamentally different from shear-induced crystallization in jammed or glassy systems. Although the computer simulations neglect hydrodynamic interactions and impose a linear flow, the results are in good agreement with the experiments. Depending on the amplitude and frequency of the oscillation, four regimes with different structures, hereafter referred to as phases, were identified: an oscillating twinned face-centered-cubic (fcc) phase, a sliding layer phase, a string phase and a phase that has not been reported previously in experiments, which we identify as tilted layers. This phase consists of lanes of particles that order in a hexagonal-like array (in the gradient-vorticity plane) which has lines of particles under an angle with the horizontal. Phases similar to the sliding layers, string phase and tilted layer phase were reported in Brownian and Molecular Dynamics simulations (systematically called string formation) but the validity of these simulations has been questioned. We demonstrate the experimental existence of these string-like phases and elucidate their structural differences in real-space.