Short- and long-time diffusion, and dynamic scaling in suspensions of charged colloidal particles

TL;DR

This study combines theory, simulation, and experiments to analyze how hydrodynamic interactions influence diffusion in charged colloidal suspensions across different time scales, revealing their role in dynamic scaling and caging effects.

Contribution

It provides a comprehensive analysis of hydrodynamic interactions' effects on colloidal diffusion using advanced simulations, theory, and experimental validation, highlighting their impact on short- and long-time dynamics.

Findings

Hydrodynamic interactions enhance diffusion at intermediate and long times.

Mode-coupling theory overestimates the slowing effect of particle caging.

Simulated scattering functions agree with dynamic light scattering experiments.

Abstract

We report on a comprehensive theory-simulation-experimental study of collective and self-diffusion in suspensions of charge-stabilized colloidal spheres. In simulation and theory, the spheres interact by a hard-core plus screened Coulomb pair potential. Intermediate and self-intermediate scattering functions are calculated by accelerated Stokesian Dynamics simulations where hydrodynamic interactions (HIs) are fully accounted for. The study spans the range from the short-time to the colloidal long-time regime. Additionally, Brownian Dynamics simulation and mode-coupling theory (MCT) results are generated where HIs are neglected. It is shown that HIs enhance collective and self-diffusion at intermediate and long times, whereas at short times self-diffusion, and for certain wavenumbers also collective diffusion, are slowed down. MCT significantly overestimate the slowing influence of dynamic particle caging. The simulated scattering functions are in decent agreement with our dynamic light scattering (DLS) results for suspensions of charged silica spheres. Simulation and theoretical results are indicative of a long-time exponential decay of the intermediate scattering function. The approximate validity of a far-reaching time-wavenumber factorization of the scattering function is shown to be a consequence of HIs. Our study of collective diffusion is amended by simulation and theoretical results for the self-intermediate scattering function and the particle mean squared displacement (MSD). Since self-diffusion is not assessed in DLS measurements, a method to deduce the MSD approximately in DLS is theoretically validated.