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 into spherical shape. This occurs because solid-like behavior of the bulk or of the surface of the droplets resists surface tension forces that drive shape recovery. Partial coalescence can significantly impact the microstructure, stability, and rheology of multiphase materials encountered in the processing of foods, cosmetics, petroleum, and polymers. We study the partial coalescence of particle-filled droplets under flow conditions. The experiments are conducted with polyethylene oxide droplets that are filled to over 50 vol% with spherical silica particles, and the resulting yield stress resists capillary forces and prevents complete coalescence. As expected, in the initial stages of shearing, irregular, partially coalesced clusters appear as the drops collide and merge. However, as shearing continues, interesting morphological developments are observed. If shear stress dominates over yield stress, the irregular structures relax into spherical, completely coalesced droplets. If yield stress dominates, irregular structures become more compact but remain non-spherical even under extended shear. We study the evolution of these structures for different blend compositions and flow conditions. These studies provide guidance on conditions under which the formation of large-scale structures may appear in particle-containing liquid-liquid mixtures.