Length scale dependence of dynamical heterogeneity in a colloidal fractal gel

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 ,τ) − 1 ∼ exp [ − (τ/τ f ) p ], with τ f ∼ 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 χ(τ, q ) of the instantaneous autocorrelation function, the analogous of the dynamical susceptibility χ 4 studied in glass formers. The amplitude of χ 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 χ.