Particle-Filled Emulsion Drops Show Flow-Induced Partial Coalescence, but Only Transiently

Partial coalescence refers to a process where two or more droplets come into contact and merge, but do not recover spherical shape. We conduct a flow-visualization study of the shear flow-induced partial coalescence of an emulsion of particle-filled drops. Experiments are conducted with poly-(ethylene oxide) drops dispersed in polyisobutylene. The drops are filled to over 50 vol % with spherical silica particles. Partial coalescence is attributable to the solid-like behavior induced by the particles inside the drops and possibly at the interface. After subjecting the emulsions to high-rate shear, the drops adopt slightly nonspherical shapes and retain them even under quiescent conditions. Subsequent shearing at lower rate causes these drops to partially coalesce into highly irregular drop shapes, and particles promote this coalescence process. But with continued shearing, the highly irregular drops gradually become rounded and approximately spherical. The reversion to rounded shape is faster at higher shear rate and at high drop loading. In contrast, at low rates or low drop loadings, drops can sustain grossly irregular shapes even when sheared for hundreds of strain units. This gradual reversion to sphericity is not driven by capillarity. Instead, we propose that the chief mechanism is that when irregularly shaped drops collide, the viscous stress in their near-contact region induces localized yielding and particle rearrangements. Thus, repeated drop collisions gradually smoothen the drops toward sphericity.