The Generation Of Microfluidic droplet
Microfluidic droplet generation. In recent years, microfluidic systems have been successfully used to generate multiphase fluids in a variety of formats. Although microdroplets may be formed in microchannels under static conditions, the adoption of continuous flow or pseudo continuous flow microfluidic formats has been shown to provide the most versatile route to high-throughput droplet generation. The generation of microdroplets within microfluidic channels can be initiated through the use of electric fields, micro-injectors and needles. However, of particular note are microfluidic systems that exploit flow instabilities between immiscible fluids to generate suspended droplets. Put simply, droplets can spontaneously form when laminar streams of aqueous reagents are injected into an immiscible carrier fluid. The two most common methods for generating droplets in this way are through the use of T-junctions and flow focusing geometries. Within a T-junction device droplets are formed by injecting an aqueous phase perpendicularly into a continuous oil phase. Here, microdroplet formation results from induced shear forces within the two phase system. Flow focusing geometries involve a slightly different configuration, in which a liquid flows in a central channel and a second immiscible liquid flows in two outside channels. The two liquid phases are then impelled to flow through a small orifice that is located downstream of the three channels. The outer fluid applies pressure and viscous stresses that drive the inner fluid into a narrow strand, which then breaks inside or downstream of the orifice (at a position defined by the capillary number) to form a small droplet. It should also be noted that microfluidic axisymmetric flow focusing geometries allow generation of droplets with reduced dimensions and size dispersions. Such an approach confines droplets to the central axis of a microchannel and shields them from shear forces and potential wetting upon contact with the channel walls.
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