Near-wall colloidal dynamics probed by evanescent-wave dynamic light scattering*

We present theoretical, numerical, and experimental investigations of evane\-scent-wave dynamic light scattering (EWDLS) in a wall-bounded colloidal suspension of spheres. The first cummulant $\Gamma_1$ representing the initial decay of the time autocorrelation function of the scattered field is expressed in terms of the hydrodynamic tensor $\mathbf{H}_w(\kappa,\mathbf{q})$ describing response of the suspension to a spatially varying harmonic force damped exponentially away from the wall. The wavelength of the harmonic spatial variation corresponds to the scattering vector $\mathbf{q}$ in the EWDLS experiments, and the exponential decay is characterized by the decay length $\kappa^{-1}$ of the evanescent wave. The hydrodynamic tensor $\mathbf{H}_w$ is evaluated using viral expansion at low densities and numerical simulations at higher densities. A complex non-isotropic structure of the tensor $\mathbf{H}_w(\kappa,\mathbf{q})$ reflects the hydrodynamic particle-wall coupling and wall-induced short range suspension ordering. Our theory and simulations agree well with the results of EWDLS experiments.