Proc. Natl Acad. Sci. USA http://doi.org/j5f (2013)

Credit: © 2013 NAS

Continuous droplet production is relatively easy: make microchannels with a suitable geometry (such as a T-junction) that allows two fluids (dispersed and carrier) to meet at appropriate flow rates, and hydrodynamic forces will do the rest. However, droplet size and flow rates depend on the viscosities and surface tensions of the two fluids. Now, Rémi Dangla and co-workers report an even simpler approach: inject the dispersed phase through an inlet with a rectangular cross-section into a reservoir with inclined top and bottom walls that is filled with the carrier fluid. Such a gradient in height causes an imbalance of curvature and pressure along the interface between the two fluids, which leads to the formation of a growing 'circular tongue' with a neck of decreasing width in the inlet channel. A droplet forms when the neck ruptures as its width reaches the height of the inlet, and self-propels into the reservoir as a result of the gradient in surface energy imposed by the inclined walls. In this strategy, droplet size depends on the confining geometry only.