Electrical control of individual droplet breaking and droplet contents extraction.

The controllable extraction of the contents from an individual droplet is very important for the analysis and further processing of droplets. Most of the available methods lack the control and flexibility over the extraction of droplet contents. Here, we present a novel electrical-based method that can selectively break a droplet and extract its contents into aqueous buffer in a controllable manner on a microfluidic device. The device consisted of two layers, in which the top layer was modified to be hydrophobic for droplet generation and the bottom layer modified to be hydrophilic for maintaining aqueous buffer. A stable oil/water interface was formed at the intersection of the two layers where the oil and aqueous buffer met. When a droplet flew through the oil/water interface, a voltage was applied in the aqueous buffer. The generated electro-osmotic flow in the aqueous buffer and the electric field facilitated the breaking of the water/oil interface between the droplet and aqueous buffer, promoting the breaking of the droplet and a transient coalescence of the droplet with aqueous buffer. During the transient coalescence, droplet contents could be extracted into aqueous buffer. The amount of the droplet contents extracted into the aqueous buffer could be controlled by varying the strength of the electric field, and the droplet still existed and could be further processed after complete extraction of the droplet contents. This method could be used for droplet analysis and may provide a way to perform complex fluid handling in a droplet.