The confounding thing about bubbles in outer space is that they don't know which way to go. It can make burping difficult for astronauts, and it can make designing next-generation space equipment nearly impossible.
Worse, in a place where gravity no longer rules, space bubbles like to gang together, forming super-bubbles the size of footballs that we must surely learn to control if we are ever to conquer the great unknown.
And if we can figure out this double-bubble dilemma, a side benefit might be better eye makeup, but more on that later in this story.
How bubbles are supposed to behave
On Earth, you can boil a liquid and be pretty darn certain that the resulting bubbles will float up. At the surface, they will separate and carry heat away. Try to boil something in space, however, and the bubbles get all confused about which way they should go.
The result is a host of practical problems for space travelers, with no known solutions. A bubble stuck in an intravenous tube can prove deadly. Fuel supply lines can be blocked. And, a badly behaved bubble in a heat-transfer system can cause a pump to shut off, overheating and destroying equipment.
So on the eve of the age when humans will live semi-permanently aboard the International Space Station and possibly travel to Mars, researchers are scrambling to figure out how to push bubbles around in micro-and low-gravity environments. One of the leading bubble pushers is Cila Herman (pictured above), who is electrifying the field, so to speak, with her new research.
Working out of Johns Hopkins University, Herman has developed a technique that uses electricity to separate and direct bubbles. The process works well enough on Earth, but on Earth, bubbles don't need any help being bubbles.
So Herman got her chance recently to move her experiments to space, or at least as close to space as one can get in an airplane.
Aboard NASA's KC-135-A "Vomit Comet," a plane that produces 30-second periods of microgravity with a series of gut-wrenching loops and hovers, Herman and her graduate students tried out their idea.
"To the best of our knowledge, we were the first to use electric fields to detach and move bubbles in microgravity," Herman said.
Why is this so difficult?
Since it's probably been a while since you studied the intricacies of terrestrial bubble dynamics, here's a quick primer:
When a liquid is in contact with a heated surface, tiny bubbles will form as the liquid vaporizes. The bubbles grow until they are large enough to separate from the heated surface. They float to the surface of the liquid because that is where the fluid pressure is the lowest. There, if all goes well, the bubble breaks the surface tension of the liquid and carries heat off into the air.
"In microgravity [the bubbles] tend to grow larger and tend to remain attached to the surface and coalesce, meaning that smaller bubbles developing on the surface tend to merge with larger bubbles to form a huge one," Herman said.
Further complicating the process is the complicated nature of fluids. How a bubble behaves involves heat flow, fluid flow, surface tension, and a range of other physical processes that scientists are only beginning to grasp.
The test
Herman's 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. A high-speed video camera documented the behavior of the bubbles, and the researchers plan to quantify their results in the coming weeks.
Eugene H. Trinh, director of the Microgravity Research Division of the Jet Propulsion Laboratory, knows first-hand the problems of space bubbles. Trinh was an astronaut aboard the space shuttle in 1992, and he says he's always been interested in how fluids behave in microgravity.
Trinh said there are other efforts underway to understand the bubble problem. This includes another project at Johns Hopkins that uses ultrasound to dislodge bubbles -- though the process has not been tested in a weightless environment.
Trinh, who said Herman's effort is probably the most versatile approach, also said the work could ultimately lead to new techniques for food preparation, and even improved cosmetics. The ability to hold a bubble and measure it, he says, would improve theories for "surfactants" -- chemicals used in the cosmetics industry.
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