Experimental study of out-of-equilibrium fluctuations in a colloidal suspension of Laponite using optical traps

We address the issue of the validity of the fluctuation dissipation theorem and the time evolution of viscoelastic properties during ageing of aqueous suspensions of a clay (Laponite RD) in a colloidal glass phase. Given the conflicting results reported in the literature for different experimental techniques, our goal is to check and reconcile them using simultaneously passive and active microrheology techniques. For this purpose we measure the thermal fluctuations of microsized Brownian particles immersed in the colloidal glass and trapped by optical tweezers. We find that several methods based on both microrheology techniques lead to consistent and complementary results and no violation of the FDT is convincingly observed either for any frequency as low as 0.25 Hz or as an increase of the effective temperature during the formation of the viscoelastic glass. Our results are supported by the study of the probability density functions of heat fluctuations between the probe particles and the suspension transferred at different timescales. Several interesting features concerning the statistical properties and the long time correlations of the particles are observed during the transition.