Drop formation: Methods and applications

From rain drops to very tiny, pico-liter sized drops produced in an ink-jet printer, drop formation is seen in a wide range of occurrences—both natural and man-made. A variety of modern applications employ drop formation to achieve different purposes. For example, small drops are created to coat a thin layer of paint on a surface in spray-painting and to form tiny pools containing reagents such as DNA in lab-on-a-chip type applications. Depending upon the final requirement, drops of different volumes and compositions are produced in these applications. Here, a short list of different methods available to make drops and their applications is included. (For a detailed review, see Basaran (2002).) In particular, techniques that are used to produce small, micron-sized (in diameter) drops are elaborated. Drop formation through jet breakup One can see a jet of water coming out of, say, a kitchen faucet just by turning on the tap. The water jet so formed breaks up into drops due to surface tension. Whereas there exists no external field in this example other than the passive ambient air, an applied electric field or an externally flowing fluid can also be employed to cause jet break up into drops. Figure 1 shows schematics of two such methods employed to produce drops. An externally applied electric fiel