Distributed evaporation of water-in-oil emulsion drops on solid surfaces

We experimentally demonstrate capillary clustering of microdroplets when a microfluidic water-in-oil emulsion drop was placed on a solid substrate. Distributed evaporation rates occur within the cluster manifested by a size gradient of microdroplets. We show that the process depends upon the oil and substrate properties. We study the evaporative dynamics of the cluster and the individual microdroplets. We identify three different evaporation regimes of the microdroplets and model the process. Unlike bare droplets, the evaporation rate varies with time when the droplet size approaches the oil height. Our findings contribute to a better understanding of droplet evaporation in emulsions and on oil-infused surfaces. Also, it may have implications for soft materials, microdroplet chemistry, and biosystems involving droplets.