Length scale dependence of dynamical heterogeneity in a colloidal fractal gel
Résumé
We use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel. The final relaxation of the intensity autocorrelation function is faster than exponential : $g_2(q,\tau)-1 \sim \exp[-(\tau/\tau_\mathrm{f})^p]$, with $\tau_\mathrm{f} \sim q^{-1}$ and $p$ decreasing from 1.5 to 1 with increasing $q$. We quantify dynamical fluctuations 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 model based on intermittent rearrangements that accounts for both the average dynamics and its fluctuations.