Microfluidic methods involve passing small volumes of fluid through microfabricated structures and manipulating these volumes to carry out biological or chemical reactions. To stage such reactions, samples, reactants, solvents, or other reagents can be encapsulated in discrete droplets having volumes on the order of nanoliters or less. A droplet is typically immersed in a carrier fluid from which it is phase-separated, and transported along with the carrier fluid through microfluidic channels. In sufficiently small channels, this transport occurs at low Reynolds number and exhibits laminar flow. Reactions can be facilitated by, for example, merging droplets (causing droplet fusion), splitting droplets (causing droplet fission), injecting material into droplets, or extracting material from droplets.
To control the movement of droplets in a microfluidic device, it can be useful to measure the velocity of droplets in real time as they pass through microfluidic channels. Similarly, for droplets subject to injection or extraction of material, it can be useful to measure changes in droplet volume on short timescales. These measurements can be fed back to systems governing the flow rate of the carrier fluid or the manipulation of droplets, allowing optimization of droplet-based reactions. Measuring changes in droplet location or size is challenging, however, because of the small dimensions of microfluidic devices and the droplets themselves. Imaging individual droplets with conventional optics requires a high level of magnification and a limited field of view. A droplet traveling through a microfluidic channel at typical velocities can traverse the field of view faster than two images of the droplet can be acquired in consecutive video frames. To obtain two or more images of the same droplet and measure a change, more sophisticated optics can be employed to expand the field of view, or a high-speed camera can be used instead of a conventional video camera. These solutions are expensive and difficult to implement.