From microdroplets to microfluidics: selective emulsion separation in microfluidic devices.

Microdroplets show great promise as a new high-throughput technology in chemistry, biochemistry, and molecular biology. Microdroplets can be generated at rates in excess of several thousands per second and accurately formulated using minute amounts of small molecules, DNA, proteins, or cells. Furthermore, integrated active elements can be used to control individual droplets. With the technology for creating, dividing, fusing, interrogating, and even sorting microdroplets already developed, one of the main problems to be resolved is how to access their contents. Droplets are naturally self-contained microreactors that prevent sample loss, diffusion, and cross-contamination, general issues that afflict traditional microfluidics. However, the isolated nature of droplets prevents physical access of their contents on-chip. Even though this does not represent a problem for many of the applications that have already been demonstrated, it limits the integration of microdroplets with other platforms. Analytical techniques such as mass spectrometry, capillary electrophoresis, and liquid chromatography have been successfully integrated with continuous-flow microfluidic devices, but their integration with microdroplets remains challenging. If the contents of microdroplets could be readily extracted on demand, the carrier fluid discarded, and the microdroplets converted into a continuous stream, microfluidic functionality could be combined with the advantages of microdroplets. In this paper we present a technology that bridges the fields of microdroplets and continuous-flow microfluidics by extracting on-chip the contents of microdroplets and incorporating them into a continuous stream. The extraction is achieved through electrocoalescence: droplets are forced to coalesce with an aqueous stream by application of an electric field across the channel. The extraction is controlled through the voltage applied at microfabricated electrodes on each side of the channel and can be performed in a continuous or discrete fashion. The discrete collection of droplets can be controlled by an external electrical signal related to the contents of the droplets. As a proof of principle, we have implemented a fluorescence intensity based detection system to control the collection of the droplets, resulting in a device capable of selectively incorporating the contents of droplets of interest to a continuous microfluidic stream. We used flow-focusing to generate microdroplets. An aqueous stream was focused between two oil streams as they pass through a junction. Shear forces make the aqueous thread break up into monodisperse droplets. Droplet size and frequency were controlled by a combination of channel dimensions and flow rates. We used a mixture of fluorous oil (FC-77) and 1H,1H,2H,2H-perfluorooctanol (70:30 by weight) as the carrier phase. The oil and aqueous flows at the flow-focusing device were adjusted to generate the desired droplet frequency, typically ranging from 10–250 Hz. The flow of the lateral aqueous phase was adjusted so an interface was held in the region between the electrodes without overflow in either direction. Figure 1a shows a scheme of a typical device where droplets flow parallel to a stream of water between two electrodes. In the absence of an electric field, the droplets are not perturbed by the presence of the aqueous stream and follow the geometrically determined flow lines. Figure 1b and c show micrographs of such a device in operation. Droplets of a dye generated at the flow-focusing device flow past the