Tracer diffusion in colloidal gels

TL;DR

This study uses computer simulations to analyze how tracer particles diffuse in colloidal gels, revealing that diffusion depends on accessible volume fraction and exhibits critical slowing down and heterogeneity related to pore sizes.

Contribution

It demonstrates that tracer diffusion in colloidal gels is primarily governed by accessible volume fraction, regardless of gel structure or tracer size, and identifies critical slowing down at a specific volume fraction.

Findings

Diffusion coefficient depends on accessible volume fraction $\,\phi_a$.

Critical slowing down occurs at $\,\phi_a\approx 0.03$ with known scaling laws.

Heterogeneity in MSD relates to pore size distribution.

Abstract

Computer simulations were done of the mean square displacement (MSD) of tracer particles in colloidal gels formed by diffusion or reaction limited aggregation of hard spheres. The diffusion coefficient was found to be determined by the volume fraction accessible to the spherical tracers ($\phi_a$) independent of the gel structure or the tracer size. In all cases, critical slowing down was observed at $\phi_a\approx 0.03$ and was characterized by the same scaling laws reported earlier for tracer diffusion in a Lorentz gas. Strong heterogeneity of the MSD was observed at small $\phi_a$ and was related to the size distribution of pores.