Physics of drop formation at a step

When a confined liquid stream surrounded by another immiscible liquid flows from a shallow channel into a deep reservoir, droplets can be generated at the entrance of the reservoir. We perform series of experiments in this geometry in a microfluidic system and compare our observations to a recent theory developed by A. Leshansky and L. Pismen. We observe that below a critical capillary number, Ca, small monodispersed droplets are formed. We give scaling explanation on their sizes. Above the critical Ca, the stream takes a quasi-steady shape feeding a large droplet formed in the reservoir. Theoretical arguments are presented explaining the transition between the two regimes and the shape of the interface, that separates the two liquids. The experiments confirm the theory at a quantitative level.