Electrowetting-controlled droplet generation in a microfluidic flow-focusing device

We studied the generation of aqueous microdrops in an oil-water flow-focusing device with integrated insulator-covered electrodes that allow for continuous tuning of the water wettability by means of electrowetting. Depending on the oil and water inlet pressures three different operating conditions were identified that shift upon applying a voltage: stable oil-water interface, drop generation, and laminar water jet formation. Full control over the drop generation is achieved within a well-defined range of inlet pressures, in quantitative agreement with a model based on the additive contributions from electrowetting and the local hydrostatic pressure at the junction. The tuning power of electrowetting is shown to increase upon device miniaturization, which makes this approach particularly attractive for flow control on the sub-micrometer scale.