Microfluidic and nanofluidic emulsification has been explored extensively in chemical engineering and biotechnology for using the emulsion droplets as microreactor, cell encapsulates, microarrays etc. [1] There is an increasing demand on the ability of tailoring the droplet size, leading to either uniform or serial sizes. Flow-focusing device is an ideal platform to produce both monodisperse and polydisperse droplets in micro-scale and nano-scale [2], but it needs a quite long respond time to change the droplets size by varying flow rates [3], which is not suitable for rapid control in droplet formation. Embedding electrical field inside the microfluidic channels provides additional capabilities to instantly control the droplets [4], and further decrease the size towards nano-scale without expensive cost.

Here, we use AC electrical field of various waveforms to perform the dynamic control on the drop formation in a flow-focusing device. The size of droplets which were generated under triangle waveform inversely changed with applied voltage. Monodisperse droplets segments were produced using the rectangle waveform, and then separated by label regimes. The droplet size in the monodisperse segments can be adjusted by the magnitude of voltage. We also found high frequency AC can be used to control the coalescence of droplets. Our technique renders high flexibility in the engineering of droplet series, and may be readily incorporated in other nano –technologies as particle synthesis, DNA/protein analysis and sorting, etc.

1. Whitesides, G.M., The origins and the future of microfluidics. Nature, 2006. 442(7101): p. 368-373.

2. Anna, S.L., N. Bontoux, and H.A. Stone, Formation of dispersions using "flow focusing" in microchannels. Applied Physics Letters, 2003. 82(3): p. 364-366.

3. Collins, J. and A.P. Lee, Control of serial microfluidic droplet size gradient by step-wise ramping of flow rates. Microfluidics and Nanofluidics, 2007. 3(1): p. 19-25.

4. Link, D.R., et al., Electric control of droplets in microfluidic devices. Angewandte Chemie-International Edition, 2006. 45(16): p. 2556-2560