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 us ually assumed to be instantaneous. Such an artificial situation is neverthe less intrinsically at variance with experiments because the quench rate is finite due to the continuous changes in thermodynamic parameters between th e initial and final states. We experimentally explore this issue in near-cr itical micellar phases of microemulsion with induced transient grating tech iques, focusing our attention on the very early stage of droplet growth, wh ere the influence of the time dependence of supersatuation is the strongest . The experiment makes use of laser-induced concentration variations to loc ally quench the mixture with two intersecting pump beams, whose interferenc e pattern optically traps the nucleated droplets on fringes. Due to the slo w mass diffusion kinetics of quenches in composition, the time-resolved ref lectivity 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 begi nning of the decay of the metastable state. By varying the amplitudes of th e control parameters (beam power, beam radii), rye are able to characterize the dynamic properties of nucleation onset during continuous quenching. Th e 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 t emporal variation of the supersaturation, where R-C and t(C) are, respectiv ely, the measured critical radius and nucleation time.