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Office of News and Information
212 Whitehead Hall / 3400 N. Charles Street
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Phone: (410) 516-7160 / Fax (410) 516-5251

April 10, 1996
CONTACT: Phil Sneiderman

Hopkins Scientists Hope to End Bubble Troubles
in Outer Space

Put a pot of water on a hot stove, and you'll soon see steam bubbles scurrying toward the surface. But if you try to boil water aboard an orbiting space shuttle, something different will occur: One huge bubble will form along the heated surface and stay there, refusing to carry heat away. Soon, portions of the heated container will dry out, and it may crack.

Bubbles can be troublesome in the gravity-free environment of space. But by using sound waves, electric fields and forced liquid currents, three scientists at The Johns Hopkins University hope to make bubbles behave in more productive ways when gravity is gone.

The research is important because bubbles play a major role in cooling and water purification, which will be crucial as space vessels spend more time in weightless conditions. To address the bubble problem, NASA's Microgravity Science and Applications Division recently awarded multi-year research grants to three faculty members in Hopkins' Whiting School of Engineering.

Aboard a space ship or an orbiting lab, rocket engines, electronic equipment and solar radiation all generate heat, which must be eliminated. Although commonly associated with cooking, boiling is a highly efficient cooling process. "Boiling is the single best way to transfer heat," said Andrea Prosperetti, a mechanical engineering professor and grant recipient.

In normal gravity, he explained, small bubbles form when a heated surface turns the liquid into vapor. Because gas bubbles are lighter than the surrounding liquid, they soon detach and float upward. As they rise, the bubbles stir the liquid, condense and transfer some of their heat to the cooler water. In weightless space, however, the bubbles don't rise because they are no longer much lighter than the water around them. Instead, one giant bubble forms and merely sticks to the hot surface. "What I am going to look at is, what happens if you put a sound field, an ultrasonic field, in the water?" Prosperetti said. "Can you remove the bubbles that way?"

The Hopkins professor, collaborating with Eugene Trinh at the California Institute of Technology, plans to send a standing sound wave, pitched at 20 kilohertz, into the liquid, bouncing it off the bottom. The sound is at the upper limits of the human hearing range but well within that of a dog. "The bubbles should pulsate, oscillate in response to the sound field," Prosperetti said. "That should help in dislodging them from the surface."

Cila Herman, an assistant professor of mechanical engineering, will attack the space bubble problem with another tool. She plans to insert electrodes and apply 30 kilovolts, forming an electric field in the liquid. "The force that is generated by the electric field should replace the buoyancy," she said. She plans to try various shapes of electrodes to see which are most effective. Herman will study the heat transfer with holographic interferometry, an innovative form of photography that uses lasers and high-speed film.

The third Hopkins scientist, Hasan N. Oguz, an associate research professor of mechanical engineering, will experiment with bubbles that do not result from boiling. He will use a hypodermic needle to push air into a liquid-filled tube in the same way that bubbles are forced into a fish tank.

On Earth, a small bubble will form at the needle's tip, then detach. "What happens in space is that initially it's very similar," Oguz said. "But this process never stops. That's the problem. You get a giant bubble, and it never detaches." To remedy this, Oguz will pump additional liquid along the sides of the needle, so that these currents will pull the bubble loose. "You need very little flow to achieve this because the tube restricts the flow around the bubbles," he said. In space, Oguz' technique could be used in water purification systems, which require the injection of oxygen.

Although initial work will be done in university labs, the Hopkins bubble experiments may later be tested in drop towers, which briefly simulate low-gravity conditions. In several years, Herman expects to check her work aboard an astronaut-training jet that flies a parabolic course to create a short period of stomach-churning weightlessness. "I'm looking for graduate students who don't get motion sickness," she said.

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