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1. Drop Formation

The simplest but perhaps most important operation in droplet-based microfluidics is drop formation. Drop formation in microfluidics can be achieved using a variety of methods, but the most common is flow-focus drop formation. In this method, the dispersed phase, which is the fluid of which the final drops are to be composed, is injected from one channel, and the continuous phase that is to surround the drops is injected from two side channels. These fluids flow into a narrow channel, where shear and pressure fluctuations generated by the flow cause drops to form. Due to the controlled flow properties of fluids at the micron scale, the drops can be formed with highly uniform size, as shown in this movie, 

from Abate et. al Biomicrofluidics (2011).

The uniformity of the drops is critical, because it sets the uniformity of the "test tubes" used to house the biological reactions. To optimize for different  reactions, drop size can be adjusted by varying the pressures with which the phases are introduced into the nozzle.

Valve-based flow focusing is another, more versatile way to make drops. This technique uses single-layer membrane valves to shape the drop formation nozzle in real time, making it wider or narrower, to adjust drop size. The advantage to this is that the drop size can be varied continuously over a fairly wide range without the need for changing flow rates, which is especially useful when prototyping new devices, as shown in this movie,

from Abate et. al, Appl. Phys. Lett. (2008).