Electric Scissors for Precise Generation of Organic Droplets in Microfluidics: A Universal Approach that Goes beyond Surface Wettability

Multifarious organic phase droplets have emerged in diverse applications such as material synthesis and biological pharmacy; accordingly, the organic droplet generation demands more on the usability and the controllability. In this article, we demonstrate a universal and simple method to actively generate organic in water (O/W) droplet in a poly­(dimethylsiloxane) (PDMS)-based microfluidics device by using both DC and AC electric fields to cut a stable layered flow of the disperse phase into dispersed droplets or liquid slugs in a precise manner. The system demonstrates the feature of ultrafast response and precise control of O/W droplet generation. The biggest advantage of the proposed approach is that it removes the necessity for surface treatment in conventional O/W droplet formation in PDMS microchannels, breaking the limits brought by the surface wettability for the first time and rendering itself to be a universal method for O/W droplet generation. We also explored the breakup of the disperse phase, catalogued them into three stages with sinusoidal AC electric fields, namely, nonbreakup, transition, and continuous breakup and mapped the voltage boundaries in these stages. To demonstrate their general application to generate organic droplets, we executed these control strategies on four typical organic fluids, each flowing with different hydrodynamic characteristics. The results showed precise cutting and tuning effects from the square and the sinusoidal wave electric fields. Our findings propose a potentially universal active formation technique for the organic-based solutions droplet and widen the applications of the microfluidics.