Diffusion of colloids at short times

TL;DR

This study investigates how electrostatic and hydrodynamic interactions influence the short-time diffusion behavior of charge-stabilized colloids, revealing distinct scaling laws and the effectiveness of an effective hard sphere model.

Contribution

It introduces a detailed analysis of short-time colloid dynamics considering electrostatic and hydrodynamic effects, highlighting non-linear scaling laws and the applicability of an effective hard sphere model.

Findings

Self-diffusion in deionized suspensions is less affected by hydrodynamic interactions.

At high electrolyte concentrations, results align with hard sphere models showing linear dependence on volume fraction.

Charged suspensions exhibit non-linear scaling laws for diffusion coefficients at low volume fractions.

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, $D^t_s$ and $D^r_s$, as function of volume fraction $\phi$ 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 $\phi$-dependence of $D^t_s$ and $D^r_s$ at small $\phi$. In contrast, for deionized charged suspensions at small $\phi$, we observe the interesting non-linear scaling properties $D^t_s\propto1-a_t\phi^{4/3}$ and $D^r_s\propto 1-a_r\phi^2$. 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.