Drop time: manipulating liquids in space.

An orbiting spacecraft can serve as a unique laboratory for studying the behavior of liquid drops. In a setting in which the effects of gravity are practically negligible, researchers can observe details of how a drop responds to various forces and how drops of different liquids interact and combine.

The U.S. Microgravity Laboratory, carried aloft last month by space shuttle Columbia (SN: 11/11/95, p.308), includes an apparatus called the drop physics module. With this equipment, a shuttle crew member can use sound waves to levitate and manipulate liquid drops about the size of golf balls.

This approach provides large spherical drops--undistorted by gravity and unaffected by container walls--as the starting point for experiments. "Whatever sample you put in there is in contact with nothing but air," says Arvid P. Croonquist of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

One set of experiments, put together by Taylor G. Wang of Vanderbilt University in Nashville and his coworkers, involved drop dynamics. The researchers studied the breakup of rapidly rotating drops and the transition to chaotic motion as sound waves forced stationary drops to oscillate.

In one case, the researchers observed a succession of extreme shape changes that went on for nearly half a minute. "This was much more rich and varied than the usual oscillations we see," Croonquist notes.

Wang and his colleagues also looked at the behavior of compound drops produced by injecting one liquid into another. They were particularly interested in observing fluid motion within the liquids as the compound drop's outer surface oscillated, causing its inner surface to move correspondingly.

Such investigations may lead to improved methods for encapsulating spherical objects--whether groups of living cells for hormone therapy or deuterium targets for fusion experiments.

In addition, the researchers observed the formation of a membrane at the interface created when two drops of different chemicals collided. "We can't say how good or bad the membrane is until we examine it in detail," Wang says. "But at first glance, it looks very pretty."

Several experiments developed by Robert E. Apfel of Yale University and his colleagues focused on the surface characteristics of liquid drops containing traces of compounds called surfactants. These additives are widely used in such products as detergents and in many industrial processes, including pharmaceutical manufacturing and oil recovery enhancement.

To get a better understanding of the molecular forces acting in the surface layer of a water drop laced with surfactant, the researchers studied the oscillations created when a drop is squeezed acoustically, then released.