Drying-Induced Packing and Coalescence of Monodisperse Oil Droplets in Aqueous Suspensions

Drying of particle-containing emulsions is a key factor in controlling the structure of films and porous materials, yet the mechanisms of droplet coalescence remain poorly understood. We hypothesize that the particle concentration modulates both droplet packing and coalescence under drying-induced compression: an effect that has not yet been systematically explored. Here, we generate monodisperse oil droplets in aqueous suspensions containing silica particles with various concentrations using droplet microfluidics and track their drying dynamics in confined capillaries. Quantitative image analysis reveals two distinct coalescence pathways: droplet-droplet and bulk-droplet, with the latter dominant regardless of particle concentration. At high particle loadings, solidification of a particulate film at the drying front promotes bulk phase propagation and hinders the dense packing of droplets. Redispersion experiments further demonstrate that catastrophic coalescence can be tuned postpreparation by adjusting particle concentration in the continuous phase. These findings suggest that dispersed-particle concentration is a key control parameter for regulating drying-induced coalescence, offering a practical route to tailor emulsions for functional films, porous materials, and demulsification processes.