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,tau) - 1 similar to exp[-(tau/tau(f))(p)], with tau(f) similar to 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.