Diffusion of colloids at short times

Abstract

We study the combined effects of electrostatic and hydrodynamic interactions (HI) on the short-time dynamics of charge-stabilized colloidal spheres. For this purpose, we calculate the translational and the rotational self-diffusion coefficients, $D_s^t$ and $D_s^r$, as function of volume fraction $\varphi$ for various values of the effective particle charge $Z$ and various concentrations $n_s$ of added 1--1 electrolyte. Our results show that the self-diffusion 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 $\varphi$-dependence of $D_s^t$ and $D_s^r$ at small $\varphi$. In contrast, for deionized charged suspensions at small $\varphi$, we observe the interesting non-linear scaling properties $D_s^t\propto 1-a_t{\varphi }^{4/3}$ and $D_s^r\propto 1-a_r{\varphi }^2$. 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.

4 pages, LaTeX, 3 Postscript figures included using epsf