Microfluidic devices are composed of networks of channels ranging from one micron to one millimeter in width. Microfluidic devices are reminiscent of electronic circuits, where the channels are the wires and the fluids flowing through them are the flowing electrons. Like the wires in a circuit, the channels in a microfluidic device are usually assembled in arrangements that allow them to perform elementary operations. For example, some arrangements are useful for forming drops, while others are useful for injecting reagents or sorting drops. Like complex electrical circuits, microfluidic devices generally consist of several of these elementary "modules" connected in such a way as to perform a more sophisticated global task. For example, a global task could be to screen a population of cells to extract the few individuals expressing a trait of interest. A microfluidic device that could do this would be composed of a cell encapsulation module, a picoinjection module to inject nutrients and other needed reagents, an incubation module to stimulate cell growth, and a detection and sorting module, to scan each drop and extract the ones with the most interesting cells. A schematic of such a device with these five modules is shown below,

Like electronic modules including resistors, capacitors, transistors, and op amps, microfluidic modules can be connected in different combinations to perform different global tasks. To understand the operation of these devices, it is most instructive to view movies of the modules in action. On these web pages, you will find high-speed camera movies of microfluidic modules in operation. As you watch these movies, bear in mind that the droplets are actually moving at thousands to hundreds of thousand per second. The cameras used to capture these movies are same cameras used to image bullets fired from a gun, operated at the same capture rates. To make the motion of the drops visible to the human eye, the movies have been slowed by typically a thousand times.