Microdroplets: Air-Triggered Dropmaking

Microfluidics can form droplets very quickly -- depending on the size and liquid properties, drops can be formed at rates of several hundred kilohertz using a technique like flow focusing. However, even so, there are upper limits to the rate of drop formation, and these can be troublesome when the fluids needed for the assay a have properties that make them difficult to disperse into drops at high rates. A solution to this is to use air bubbles to stimulate drop formation: at very high flow rates, the dispersed phase will form stable jets rather than breaking into drops; however, by injecting air bubbles beside the jet and confining both in a narrow channel, the jet can be induced to break into drops. Provided the bubbles are injected periodically, so that they are equally spaced, the drops that are formed will be monodisperse. This typically allows the drop formation to be increased by tens to hundreds of times, as illustrated in this movie:

In this technique, the size of the resulting drops depends on the spacing of the air bubbles; a longer spacing partitions longer sections of the jet between the bubbles, resulting in larger drops. A simple way to vary drop size is thus to adjust the bubble injection frequency, which can be achieved by varying air pressure, as shown in this movie: