Short-time Brownian motion in colloidal suspensions: Experiment and simulation

We used dynamic light scattering (DLS) and computer simulations based on the fluctuating lattice Boltzmann equation (LBE) method to study the short-time Brownian dynamics of colloidal particles that interact like hard spheres. The dynamics are characterized by a Q-vector-dependent diffusion coefficient ${\mathit{D}}_{\mathit{S}}$(Q). Using DLS, we have mesaured ${\mathit{D}}_{\mathit{S}}$(Q) in the vicinity of the main (first) peak in the structure factor S(Q) for samples of poly-methylmethacrylate particles at volume fractions φ ranging from dilute up to the disorder-order transition (i.e., crystallization at φ=0.494). In addition we have determined the short-time self-diffusion (${\mathit{D}}_{\mathit{S}}^{\mathit{S}}$) and collective-diffusion ${\mathit{D}}_{\mathit{S}}^{\mathit{C}}$ coefficients. We have extracted the same quantities from simulations of equilibrium configurations of hard spheres using a fluctuating lattice Boltzmann equation method for the fluid phase coupled to Newtonian mechanics for the colloidal particles. For all samples studied, close quantitative agreement is found between the results of the DLS experiments and the LBE simulations.