Effects of partial miscibility on drop-wall and drop-drop interactions

The effect of the mutual diffusion of two polymeric phases on the interaction and coalescence of two nearby drops in quiescent conditions is investigated for two partially miscible systems, differing in the miscibility of the components. Transient interfacial tension measurements show that the polybutene (PB)/polydimethylsiloxane (PDMS) system is highly diffusive in terms of diffusing low-molecular weight species, while the polybutadiene (PBD)/PDMS system is less miscible. Drops of the highly diffusive PB/PDMS system at distances closer than their equivalent radius attract each other and coalesce with a rate that, in the last stage of the coalescence process, is the same for all drop combinations. For the slightly diffusive PBD/PDMS system, no coalescence occurs, and, in contrast, repulsion between the drops is observed. These phenomena are qualitatively explained in terms of the overlap of diffuse layers formed at the drop surfaces of two, close enough drops, yielding concentration gradients that cause gradients in the interfacial tension. These gradients yield Marangoni stresses that induce flow leading either to attraction or repulsion. To determine whether Marangoni stresses are strong enough to displace a drop in quiescent conditions, single drops of PB and PBD are placed in a PDMS matrix in the vicinity of a wall. A lateral drop motion toward the wall is observed for the highly diffusive PB/PDMS system only, while PBD drops do not move. The diffuse-interface model is considered as a good candidate to capture these phenomena described since it couples the mutual diffusion of the low-molecular weight component with both drop and matrix, while including hydrodynamic forces. The presented numerical simulations indeed show a diffusion-induced macroscopic motion that qualitatively reproduces the experimental phenomena observed and support our interpretations.