Kinetics and morphology of phase separation in fluids: The role of droplet coalescence

We aim to relate the kinetics of phase ordering to the morphology of domains pattern. We consider the late stages of phase transition where coarsening is coalescence-limited, in the vicinity of a liquid-liquid or gas-liquid critical points so as to benefit from its universality features. The volume fraction of the phases [Phi = vapor/(liquid plus vapor)] is varied by performing thermal quenches below the phase coexisting curve. Gravity effects are suppressed by carrying out experiments under reduced gravity and/or with density-matched binary liquids. We observe for the particular volume fraction Phi(H) = 32+/-3% a sharp transition between slow growth (lengthscale similar to time(1/3), drop pattern, nose coalescence) and fast growth (lengthscale similar to time, interconnected pattern, dimple coalescence). We show from numerical and scaling analyses that a threshold value Phi(H) approximate to 30% naturally comes out from the interplay between Brownian diffusion and hydrodynamics interactions.