Length scale dependence of dynamical heterogeneity in a colloidal fractal gel

TL;DR

This study investigates the length scale dependence of dynamical heterogeneity in a colloidal fractal gel using light scattering, revealing intermittent rearrangements as the primary cause of slow dynamics and quantifying fluctuations through a generalized susceptibility.

Contribution

It introduces a model linking intermittency to length scale dependence of dynamics and fluctuations in colloidal gels, challenging previous ballistic process assumptions.

Findings

Decay of autocorrelation functions follows a stretched exponential form.

Dynamical fluctuations grow linearly with wavevector q.

Intermittent rearrangements dominate the slow dynamics.

Abstract

We use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel. The final decay of the average intensity autocorrelation function is well described by $g\_2(q,\tau)-1 \sim \exp[-(\tau/\tau\_\mathrm{f})^p]$, with $\tau\_\mathrm{f} \sim q^{-1}$ and $p$ decreasing from 1.5 to 1 with increasing $q$. We show that the dynamics is not due to a continuous ballistic process, as proposed in previous works, but rather to rare, intermittent rearrangements. We quantify the dynamical fluctuations resulting from intermittency by means of the variance $\chi(\tau,q)$ of the instantaneous autocorrelation function, the analogous of the dynamical susceptibility $\chi\_4$ studied in glass formers. The amplitude of $\chi$ is found to grow linearly with $q$. We propose a simple --yet general-- model of intermittent dynamics that accounts for the $q$ dependence of both the average correlation functions and $\chi$.