Through experiments aboard a jet that simulates weightlessness, a Johns Hopkins engineer has shown that an electric field can kick loose the bubbles that stubbornly refuse to move in outer space. The research, sponsored by NASA, is important because managing bubbles is crucial to the safe and efficient cooling of power generators, propulsion units and life support systems in space.
Cila Herman, an associate professor of mechanical engineering, had earlier shown that electric fields could move bubbles in an earthbound lab. In mid-October, Herman and two of her graduate students tested the theory in a weightless environment.
Herman and her students conducted experiments for four days aboard a KC-135A turbojet that flies parabolic arcs to produce periods of zero gravity, each lasting almost 30 seconds. The NASA jet is officially called the Weightless Wonder, but it is also dubbed the "vomit comet" because it often afflicts its passengers with motion sickness. About 30 to 40 arcs took place during each two-and-a-half-hour flight, allowing the Hopkins engineers many opportunities to refine and change the parameters of their experiments.
The periods of weightlessness provided valuable insights into the unusual ways that bubbles behave in outer space. If you heat a pan of water in normal gravity, small bubbles form and scurry toward the surface because they are lighter than the surrounding fluid. But when gravity is gone, the vapor does not rise. Instead, it remains in the area where it entered the liquid, forming one or more large stationary bubbles. On Earth, rising bubbles carry heat away from a hot surface. But in space, this vapor clings to the heated surface. It may cause the surface to overheat and crack or cause the heating unit to burn out. Bubbles that refuse to move in zero gravity also can clog critical fluid supply lines.
Herman proposed that electric fields could jar bubbles loose in zero gravity and move them away from the surface where they formed. Research in her heat transfer lab at the School of Engineering supported this theory, and the NASA flights gave her the chance to test it in conditions that mimicked outer space. Her team constructed a transparent cube and filled it with an electrically insulating fluid. Inside, the researchers had installed a cylindrical high-voltage electrode and a ground electrode. When the NASA jet created a period of weightlessness, Herman and her students injected air into the chamber to form bubbles within the electric field. "The electric field generated a force analogous to gravity, causing the bubbles to detach from the orifice where they formed and to move toward one of the electrodes," Herman explains. Team members used a high-speed video camera to carefully document the behavior of the bubbles, and they plan to quantify their results in the coming weeks.
Preparing for weightlessness proved to be an interesting challenge. Before boarding the NASA jet in Cleveland, the researchers securely fastened the testing equipment to metal racks. But they overlooked a crucial part of the computer they brought aboard to control some of the experiments. "When zero gravity begins, everything that's not tied down floats up and away--including the computer mouse," Herman says. "We found that out during our first flight." The researchers replaced it with a trackball that was attached to its base.
"Practical microgravity is a lot different from theoretical microgravity," Herman says. "It was humbling to see how nature really operates." For example, a chamber that smoothly collected excess air in the lab on Earth did not operate very well during the abrupt shifts from high gravity to low, leaving a large bubble trapped in the test cell. Herman and her students plan to modify the equipment to correct this problem before taking it on follow-up flights early next year.
Although they had to contend with a few bouts of nausea, two students, both pursuing their doctorates in mechanical engineering, enjoyed the opportunity to assist Herman in zero gravity. "It was great!" said Steven Marra of Lewisburg, Pa. "It was something I always wanted to do." The other student, Gorkem Suner of Turkey, added, "Parts of it were not so much fun, but most of it was. I'll be happy when we get all the data analyzed. My thesis depends on it!"
Hans-Martin Ruf, a visiting scholar from Germany, accompanied the other three researchers to Cleveland and provided support on the ground. Herman's research team also included doctoral students John Chou of New York City and Ozan Tutunoglu of Turkey, and university staff members Bill Darling and Curt Ewing. Research funding was provided by NASA, which had earlier nominated Herman for a Presidential Early Career Award for Scientists and Engineers. That honor, which she received in 1997, also provided funding for the microgravity experiments.