LONG-TIME DYNAMICS OF CHARGED COLLOIDAL SUSPENSIONS - HYDRODYNAMIC INTERACTION EFFECTS

For charge-stabilized suspensions, we study the influence of hydrodyna mic interactions (HI) and electrostatic interactions on the dynamic st ructure factor S(q,t), and on the long-time self-diffusion coefficient D-s(L). Both types of interactions give rise to memory effects, which lead to a slower and non-exponential decay of S(q, t), and which caus e D-s(L) to be smaller than the short-time self-diffusion coefficient D-s(S). A global measure of the non-exponential decay of S(q, t) is th e non-exponentiality factor Delta(q), which can be determined experime ntally by dynamic light scattering. We derive microscopic expressions for the one-particle irreducible memory functions associated with Delt a(q) and D-s(L), based on the generalized Smoluchowski equation and on a projection operator method developed recently by Kawasaki. HI are a ccounted for by a far-field expansion of the two-body hydrodynamic mob ility tensors, by including the leading terms. We calculate Delta(q) a nd D-s(L) by constructing a novel mode-coupling approximation scheme w hich accounts for HI, and we extend the theory to moderately polydispe rse suspensions. Our results for the measurable non-exponentiality fac tor compare well with available experimental data. The effect of HI in charge-stabilized colloids is to reduce the non-exponentiality of S(q , t), and to cause an unexpected enhancement of D-s(L). Our calculatio ns clearly demonstrate the importance of HI at volume fractions even a s low as 10(-3).