Morphology transition observed in a phase separating fluid

The relation between morphology and kinetics of phase ordering in fluids is studied near a critical point (with benefit of universality) and under red uced gravity. The volume fraction phi of the phases governs the phase order ing. It is varied in a SF6 sample (density 0.85% off critical) by performin g thermal quenches below the coexistence curve (range: 50-2490 mu K). We ob serve for phi(H) = (29+/-3)% a transition from a pattern of isolated drops (phi<phi(H)), whose distance grows as (time)(1/3), to a pattern of intercon nected drops (phi>phi(H)), with distance growing as (time)(1). The transiti on separates the regions where collisions of droplets, which dominate the d ynamics, are due to Brownian diffusion or to hydrodynamic interactions. [S1 063-651X(99)12502-9].