Fluctuations and noise in time-resolved light scattering experiments: Measuring temporally heterogeneous dynamics

We use Time Resolved Correlation (TRC), a recently introduced light scattering method, to study the dynamics of a variety of jammed, or glassy, soft materials. The output of a TRC experiment is c(I) (t, tau), the time series of the degree of correlation between the speckle patterns generated by the light scattered at time t and t + tau. We characterize the fluctuations of c(I) by calculating their Probability Density Function, their variance as a function of the lag tau, and their time autocorrelation function. The comparison between these quantities for a Brownian sample and for jammed materials indicate unambiguously that the slow dynamics measured in soft glasses is temporally heterogeneous. The analogies with recent experimental, numerical and theoretical work on temporal heterogeneity in the glassy dynamics are briefly discussed.