Electrowetting-based drop generation in microfluidic flow focusing device

Continuous-flow microfluidic systems based on closed channels and electrowetting(EW)-based microfluidic systems using individual drops are the most widely used microfluidic platforms. So far, these two approaches have been considered independently. For two-phase flows, however, integrating the two technologies into one allows to combine the advantages of both worlds: (i) high throughput (from channel-based systems) and (ii) precise control over each individual drop (from EW).

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Here, we integrate electrowetting functionality into a microfluidic flow focusing device (FFD) to demonstrate the superior control of drop generation that can be achieved using electric fields. In addition to the pressures (or flow rates) of the continuous and the dispersed phase, the electrical voltage appears as a third control parameter for the drop generation. In contrast to pressures and flow rates, the voltage (and thus the induced variation of the wettability) can be switched over a wide range at high speed (MHz if necessary).

We demonstrate that the drop size and drop generation frequency can be tuned with much more precision than hydrodynamically, accessing also novel drop generation scenarios with variable charge (such as conical spray mode with charged micrometer-sized drops). In this regime the monodisperse tiny drops with a few microns in diameter or smaller can spray out off orifice with a high generation frequency in the order of kHz. These results illustrate the power of combining the continuous flow and EW-based approaches to microfluidics.