Once the liquid comes out of the nozzle, it breaks up into droplets. This is called "primary atomization." The exact details of the process in high-speed flows are unknown. I, and many others, have worked hard to make reasonable estimates of what happens. Once formed, the droplets can break up further, or they can collide with other drops. The drops also can evaporate and exchange heat with the surrounding gas. Turbulent eddies in the gas disperse drops, too.

Some other important applications of two-phase flow are medical aerosols and ink jet printing. In the medical field, it is important that we can produce and isolate drugs that can be inhaled for rapid utilization by the patient. However, this is only possible using advanced aerodynamics that produce a limited size range of particles, from one to five microns. Ink jet printing, though less essential to human existance, is a very interesting challenge. The inks are complex fluids that we must control precisely for fast, high-quality prints.

We acknowledge the financial support of: the National Science Foundation, NASA, Office of Naval Research, Department of Energy, General Motors, Caterpillar Inc., Kodak, Fluent Inc., United Technologies Corp., Air Force Research Laboratory, and the Army Research Office.