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Droplet merger

Picoinjection is generally the preferred method for adding reagents to drops, because it is robust and allows the volume injected to be controlled with a high degree of precision. However, an alternative way to add reagents is to simply merge two drops drops together; the resultant drop is composed of fluid from the original two drops, as shown in this movie:

Merger is achieved by flowing pairs of drops through a region of high electric field, which destabilizes the surfactants coating the drops, causing them to merge. Since the drops are typically flowing at rates of many kilohertz, this requires that the periodicities of the two droplet streams be perfectly matched and locked in the optimal phase; this requires very precise control of the flow rates. Since flow rates tend to drift over time, however, there will be periods over which the frequencies drift apart and, consequently, drops do not merge. This is compounded when more than one reagent is to be added to the drop, since this requires synchronization of several drop streams. This is further exacerbated when working with reinjected emulsions, which tend to flow with less regularity than drops formed on the fly. Consequently, picoinjection, particularly when working reinjected emulsions and adding more than one reagent, is normally the method of choice.