Late stage kinetics of a phase separation induced by a cw laser wave in binary liquid mixtures

cw laser waves can be used to locally induce phase transitions. We investig ate experimentally and theoretically a laser-driven liquid-liquid phase sep aration in a micellar phase of microemulsion and analyze its late stage kin etics. The medium is optically quenched in composition in the metastable re gion of the phase diagram. Two processes can lead to these concentration va riations: electrostriction and thermodiffusion. The first originates from i nduced dipolar couplings in a field gradient. The second corresponds to a v ariation in concentration driven by a small thermal gradient. Since the nuc leated droplets are optically trapped by the beam, we show that it becomes possible to experimentally analyze an academic situation, i.e., the diffusi on-driven growth of a single droplet in compensated gravity. The late stage of this kinetics can be divided into two parts, a bulk behavior and a regi me controlled by the finite transverse size of the beam. The bulk regime is totally analogous to that observed in classical situations (R proportional to t(1/3), where R is the droplet radius and t the time), and the scaling of the amplitudes in terms of reduced length and time scales is in total ag reement with the expected behaviors for fluids belonging to the same Ising universality class. Moreover, the Gaussian beam behaves as an optical bottl e with "soft walls" in which the absence of rigid boundaries, and thus of w etting couplings, allows an intrinsic description of the influence of finit e-size effects on the kinetics. The beam size breaks the dynamic universali ty when the growing domains start to feel it. We experimentally investigate the resulting slowing down, and a diffusion-driven model of the growth ins ide a laser wave is built for comparison. The good agreement observed for t he bulk regime and during its modification induced by finite-size effects o pens a promising field for the development of this new application of laser waves to control out-of-equilibrium liquid mixtures. [S1063-651X(99)10005- 9].