Diffusion of colloids at short times

Abstract

We study the combined effects of electrostatic and hydrodynamic interactions (HI) on the short-time dynamics of chargestabilized colloidal spheres. For this purpose, we calculate the translational and the rotational selfdiffusion coefficients, D ^t_s and D ^r_s , as function of volume fraction φ for various values of the effective particle charge Z and various concentrations n_s of added 1−1 electrolyte.

Our results show that the selfdiffusion coefficients in deionized suspensions are less affected by HI than in suspensions with added electrolyte. For very large n_s, we recover the well-known results for hard spheres, i.e. a linear φ-dependence of D ^t_s and D ^r_s at small φ. In contrast, for deionized charged suspensions at small φ, we observe the interesting non-linear scaling properties D ^t_s ∝ 1−a_tφ^4/3 and D ^r_s ∝ 1−a_rφ². The coefficients a_t and a_r are found to be nearly independent of Z. The qualitative differences between the dynamics of charged and uncharged particles can be well explained in terms of an effective hard sphere (EHS) model.