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We describe a method for creating discrete femtolitre-scale water-in-oil droplets on demand, based solely on a geometrically induced reduction in oil/water interfacial area at microfabricated junction orifices. This on-demand generation method is driven by self-shear of droplets due to interfacial tension induced forces resulting from a localized transition in microchannel height. The magnitudes of shear stresses involved appear to be significantly less than the shearing instabilities used to split off daughter droplets from aqueous mother plugs at microfabricated junctions in continuous water-in-oil segmented flows, which implies that this method may be better suited for studying biochemical reactions and reaction kinetics in droplets of decreased volume without loss of chemical reactivity due to redistribution of surfactant density used to passivate the oil/water interface. Predictable droplet generation rates under constant pressure conditions or the gated formation of one, two or more droplets at a time with fixed pressure pulses have been demonstrated in a similar manner to active on-demand droplet generation strategies, but with a simpler system not needing actuation and sensing equipment beyond a pressure regulator.
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