May 1, 2003Click on either image to view the complete 3 Mb Quicktime movie.

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May 1, 2003: "We've been doing some repair procedures up here using a soldering iron," says ISS science officer Don Pettit, "and that's what inspired this episode of Saturday Morning Science."

Pettit squirted some water onto his hot soldering iron. The water wrapped itself around the barrel and stuck there--held by molecular adhesion. Surface tension contained the fluid in a quivering blob. Strange? Pettit and his crewmates have learned to expect such behavior from weightless water.

Next the water did something surprisingly familiar: It began to boil, "and it looked like boiling water on Earth," says Pettit.

This is surprising because the commonplace appearance of boiling water is a side-effect of gravity. Bubbles of hot steam are lighter than water. They rise. Convection stirs the fluid. A turbulent rolling motion develops.

What caused the lookalike motions in space?

"We're not sure," says fluid physicist Frank Harlow of the Los Alamos National Laboratory, "but here's one possibility: bubbles formed near irregularities on the surface of the soldering iron. (These are called 'nucleation sites.') The pressure of hot water near the barrel pushed the bubbles radially outward. Bubbles bumped together; some rebounded, others coalesced. The overall effect resembles boiling water on Earth."

Pettit used only a few milliliters of water for his demonstration. "That's all I wanted to deal with," he says. Bigger pools of water have been boiled onboard the space shuttle. In those experiments, a single big bubble of steam grew around the heating element--not at all like the many small bubbles on Pettit's soldering iron.

The physics of boiling is important. We boil fluids to cook, to operate power plants, to cool engines and much more.... Fluid physicists have written computer programs--"large ones," says Harlow--to predict the complex interactions between boiling liquids and steam-filled bubbles. "We're always looking for ways to check these codes in new situations--like weightless water on the tip of a soldering iron," he adds. "Don has shown that we still have a lot to learn."