Droplet Coalescence in Confinement

Droplet coalescence has been extensively studied in both unconfined settings and in strongly confined geometries, but the intermediate regime where droplets merge adjacent to a single confining boundary remains poorly understood. Here we examine how geometric confinement and surface coating influence the coalescence of Dextran-rich droplets dispersed in a polyethylene (PEG)-rich continuous phase. Droplets were confined between glass slides separated by 76–120 µm spacers, with the bottom surface coated by either a fluorinated oil or a PEG polymer brush and the upper surface PEG-coated to prevent wetting. Confocal fluorescence imaging captured droplet shape evolution during merging. Image analysis provided the long-axis length and neck radius as functions of time, revealing size-dependent deformation and relaxation behavior. The characteristic relaxation time τ shows a continuous trend, increasing from Stokes-like behavior for unconfined droplets to slower relaxation under partial confinement and on high-friction surface coatings. These results establish that the confinement and boundary conditions are key to predict droplet coalescence, which has relevance for emulsions, biomolecular condensates, and artificial cells.