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

Abstract

We address the issue of the validity of the fluctuation dissipation theorem and the timeevolution of viscoelastic properties during ageing of aqueous suspensions of a clay(Laponite RD) in a colloidal glass phase. Given the conflicting results reported in theliterature for different experimental techniques, our goal is to check and reconcile themusing simultaneously passive and active microrheology techniques. For this purpose wemeasure the thermal fluctuations of microsized Brownian particles immersed in thecolloidal glass and trapped by optical tweezers. We find that several methods based on bothmicrorheology techniques lead to consistent and complementary results and noviolation of the FDT is convincingly observed either for any frequency as low as0.25 Hz or as an increase of the effective temperature during the formation of theviscoelastic glass. Our results are supported by the study of the probability densityfunctions of heat fluctuations between the probe particles and the suspensiontransferred at different timescales. Several interesting features concerning the statisticalproperties and the long time correlations of the particles are observed during thetransition.