Demonstration of an impact based on-demand droplet generator.

(A) Schematic of droplet generator operation. A cantilever beam plucked by a gear tooth induces a pin to impact the channel, in correspondence to punched holes. (B) Photomicrographs of on-demand impact droplet generator at a flow-focusing geometry. Water droplets (green food coloring in DI water) were dispensed in mineral oil (viscosity 15cP, surfactant 2%v/v Tween 20). (C) Timing control between arrival of punched hole, corresponding impact of the pin and the resultant single droplet formation (inset depicts magnified view with time delay between the three events). (D) High-speed imaging of droplet generation reveals an impact jet that forms in the first 2ms of the impact. The jet is quickly destabilized with formation of a narrow thread that breaks into a single droplet. Sequence of images depicts the entire operation ending with a single droplet formation over a short period of only 55ms.