The mechanism of droplet formation in microfluidic EDGE systems

Edge-based droplet generation (EDGE) emulsification, which produces multiple, monodispersed droplets simultaneously at one droplet forming unit (introduced recently by our group), is studied in more detail with high-speed imaging, computational fluid dynamics and geometric modeling as research tools. Complete filling of the plateau, essential for multiple droplet generation, is caused by the difference in pressure needed to invade the plateau and the pressure necessary to start droplet formation. In addition, the droplet formation unit has the highest hydrodynamic flow resistance in the system, which ensures oil supply and promotes plateau filling. The locations at which droplets are formed were found to be very evenly spaced with about 25 times the plateau height between them. CFD simulations showed that there is a scaling relation with the viscosity ratio, similar to that of microchannel systems. At high viscosity ratios, a minimal droplet diameter is found, which is typically 5.5-6.5 times the plateau height and can be estimated from geometric considerations only.