Short-time rotational diffusion in monodisperse charge-stabilized colloidal suspensions

We investigate the combined effects of electrostatic interactions and hydrodynamic interactions (HI) on the short-time rotational self-diffusion coefficient D-s(r) in charge-stabilized suspensions. We calculate D-s(r) as a function of volume fraction phi for various effective particle charges and various amounts of added electrolyte. The influence of HI is taken into account by a series expansion of the two-body mobility tensors. At sufficiently small phi this is an excellent approximation due to the strong electrostatic repulsion. For larger phi, we also consider the leading hydrodynamic three-body contribution. Our calculations show that the influence of the HI on D-s(r) is less pronounced for charged particles than for uncharged ones. Salt-free suspensions are particularly weakly influenced by HI. For these strongly correlated systems we obtain the interesting result D-s(r) = D-0(r)(1 - a(r) phi(2)) for small phi. Here D-0(r) denotes the Stokesian rotational diffusion coefficient, and a(r) is a positive parameter which is found to be nearly independent of the particle charge. The quadratic phi-dependence can be well explained in terms of an effective hard sphere model. Experimental verification of our theoretical results for D-s(r) is possible using depolarized dynamic light scattering from dispersions of optically anisotropic spherical particles.