The process of pattern formation and droplet coarsening has been studied for ternary polymeric fluids in which phase separation was induced by solvent evaporation. For a particular range of solvent evaporation rates and thicknesses of liquid layers, ordered hexagonal patterns were formed at the liquid film-air interface. We ascribe this effect to Bénard-Marangoni convection induced by solvent evaporation and estimate conditions generating periodic two-phase structures in polymeric films. Droplet coarsening rate featured a crossover from R ϳ t 0.89 to R ϳ t 0.67 coinciding with the onset of convection and was explained by convection-induced stabilization of droplets against coalescence.[S0031-9007(98)07184-1] PACS numbers: 61.25.Hq, 47.54. + r, 61.41. + e, Under particular conditions, patterns with a high degree of order and symmetry emerge in phase-separating polymeric fluids on a length scale significantly exceeding molecular dimensions [1,2]. For multicomponent liquids undergoing phase separation due to solvent evaporation, it was suggested that Bénard-Marangoni convection [3] may be a source of ordered patterns. Indeed, cooling of the top surface due to the solvent removal is similar to heating of a fluid film from below in a sense that it generates temperature gradient in a liquid layer. Fluctuations of temperature at the liquid-air interface result in local surface tension variations enhanced by the upward flow of the warmer liquid from the bulk. In a steady state, surface-tension-driven convection arises in the fluid layer.The manner in which the dynamics of phase separation and structure evolution are influenced by convection has not been studied although it is known that hydrodynamic effects play a significant role in droplet coarsening. For the cooperative action of Brownian motion and hydrodynamic interactions on the domain growth, Siggia [4] showed that hydrodynamic effects accelerate coalescence-induced coarsening to R ϳ t l , where R is a droplet mean radius, and t is a coarsening time. Furukawa [5], using coupled Navier-Stokes and Cahn-Hillard equations, predicted for 3D systems with a high Reynolds number two new growth modes that originated from a balance of the inertial forces and interfacial tension, i.e., R ϳ t n with n 1 and n 2 3 . Experimental studies of phase-separation dynamics in a gravity-dominated regime showed that flow can either enhance or suppress droplet growth, depending on a balance of viscous and sedimentation forces [6].It is reasonable to expect that involvement of droplets of a new phase in convection of the intervening liquid would generate coarsening dynamics different from that predicted and observed in a flow-dominated regime.In Bénard-Marangoni convection, pattern formation is governed by a balance of the surface-tension-driven forces and dissipation due to the thermal diffusion and the frictional action of viscosity [3]. The competition of these factors is expressed by a Marangoni numberwhere ͑≠g͞≠T ͒ is the temperature derivative of the surface tension g; DT is the dif...

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