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6. Incubation

A mundane but almost always necessary module is droplet incubation. This is needed whenever there is a step in an assay that requires the reagents to be held for fixed time at a fixed temperature. Droplets are normally incubated in "delay lines," which are merely long channels, as shown in this movie:

In this example, the droplets flow single-file through a relatively narrow delay line. Due to the high speed of the drops, typically kilohertz, this delay line stores the drops for only fraction of a second, far too slow for most biological assays. A simple way to increase incubation time is to lengthen of the delay line; however, this is impractical for even moderate delays, because very long lines are needed. For example, achieving a delay of only 1 min with a channel like the one above would require a channel length of 40 m. Such a long and narrow channel would require a pressure of over a megapascal -- the same pressure at the bottom of the ocean.

Long, narrow channels thus make for poor delay lines. A better way to make delay lines is to simply widen the channel. If the channel diameter is increased by a factor of 10, for example, the length needed to hold the same number of drops is decreased by a factor of 100, and the pressure needed to pump the drops at the same frequency would be reduced by a factor of 10^-6. In real world numbers, this would shorten the above delay line to only 40 cm, requiring a pumping pressure of only a few Pascal -- which is about the pressure needed to pump fluid through a straw, and well within the operating range of microfluidic devices.