Cooking up bubbles make tiny pumps.

Imaginative aircraft designers envision that the skin of future airplane wings will ripple with moving swells and dimples like the surface of a wind-blown lake. Millions of minuscule lifting devices, each controlling the height of a patch of wing, could rapidly adjust wing contours in response to changes in the fast-moving currents of air. By preventing turbulence in those currents, the fluctuating wing shapes might allow extraordinary maneuverability, stability, and fuel economy.

To achieve this vision, aircraft makers will require a fast, inexpensive, and reliable way to control and power those millions of tiny lifting machines. Engineers at Johns Hopkins University in Baltimore, Md., report that they are developing diminutive, bubble-powered pumps that may be good candidates for the job. The pumps would work by firing a series of miniature heaters in sequence along a capillary tube. This action would create vapor bubbles, whose well-timed swelling and shrinking would drive the fluid along.

"Bubbles are inherently fast, and they also pack a whole lot of energy per unit area," says Andrea Prosperetti, leader of the Johns Hopkins project, which is funded by the Air Force Office of Scientific Research. Bubbles also lack mechanical moving parts that can wear or break, he adds.

So far, the team has proved that it can make bubbles, a fraction of a millimeter in size, in water by sending current through a resistance heater made from a filament of gold affixed to silicon by standard computer-chip-making techniques. The researchers next plan to test computer models of bubble action by using a 100-micrometer-diameter capillary tube covered with Plexiglas so they can watch their bubbles work.