Nucleation and early stage growth in phase-separating liquid mixtures under weak time-dependent supersaturation

In the standard description of phase separation, quenching from an initial equilibrium state to a final metastable state in the two-phase region is usually assumed to be instantaneous. Such an artificial situation is nevertheless intrinsically at variance with experiments because the quench rate is finite due to the continuous changes in thermodynamic parameters between the initial and final states. We experimentally explore this issue in near-critical micellar phases of microemulsion with induced transient grating techiques, focusing our attention on the very early stage of droplet growth, where the influence of the time dependence of supersatuation is the strongest. The experiment makes use of laser-induced concentration variations to locally quench the mixture with two intersecting pump beams, whose interference pattern optically traps the nucleated droplets on fringes. Due to the slow mass diffusion kinetics of quenches in composition, the time-resolved reflectivity of a third probe beam on the resulting droplet grating allows us to determine the mean nucleation time and the mean quench depth at the beginning of the decay of the metastable state. By varying the amplitudes of the control parameters (beam power, beam radii), rye are able to characterize the dynamic properties of nucleation onset during continuous quenching. The results are interpreted in the light of very simple scaling arguments. We show ill particular that R-C proportional to t(C)(1/3) for a weak linear temporal variation of the supersaturation, where R-C and t(C) are, respectively, the measured critical radius and nucleation time.