A liquid mixture close to its critical demixing point is a perfect example of critical bath where the thermal fluctuations are enhanced and can be tuned by slight changes in the temperature. Although studied in equilibrium by micromanipulation techniques, with results going from Casimir-like forces or the effect of the increasing correlation length in bacterial motility, very little has been done out of equilibrium. Here, we will present our results on the relaxation dynamics of optically trapped particles coupled to critical baths. Our experiments have the novelty of using local laser induced heating for producing temperature quenches. We report on the microrheology of the critical fluid at different critical distances as well as in the relaxation of critical Casimir forces after temperature or distance quenches.
The electric thermal noise has been measured in two aging materials, a colloidal suspension (Laponite) and a polymer (polycarbonate), presenting very slow relaxation towards equilibrium. The measurements have been performed during the transition from a fluid-like to a solid-like state for the gel and after a quench for the polymer. For both materials we have observed that the electric noise is characterized by a strong intermittency, which induces a large violation of the Fluctuation Dissipation Theorem (FDT) during the aging time, and may persist for several hours at low frequency. The statistics of these intermittent signals and their dependance on the quench speed for the polymer or on sample concentration for the gel are studied. The results are in a qualitative agreement with recent models of aging, that predict an intermittent dynamics.
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