Bizarre Boiling

The International Space Station uses a "2-phase" cooling system in which ammonia changes from liquid to vapour and back, which involves boiling. Engineers designing the ISS cooling system used information gleaned from microgravity boiling experiments.

"The phenomenon of boiling is so complex that most of our understanding is empirical, rather than based on the solutions to fundamental equations," Chiaramonte says.

In the free-fall of orbit, boiling is simpler than it is on Earth. Weightlessness effectively removes two of the variables in boiling - convection and buoyancy. This difference explains why boiling liquids behave so differently in space. It also provides a powerful tool for scientists who want to unravel the tangled physics of boiling.

"As an example, imagine you were trying to study the Earth, which has such complex ecosystems. You would also want to look at a simpler planet with fewer variables. One thing space does for us is simplify the problem that we're studying," Chiaramonte says.

When a pool of liquid is heated on Earth, gravity causes hotter regions in the liquid to rise, and cooler, more dense parts to sink - a process called "convection." This motion spreads the heat around inside the liquid. Once it begins to boil, buoyancy sends bubbles hurling upward, creating a "rolling boil."

All of this motion within the liquid makes the physics of the situation much more complex.

Without convection or buoyancy, the process unfolds differently. Heated fluid doesn't rise, and instead just sits next to the heater surface and continues to get warmer. Regions of liquid away from the heater remain relatively cool. Because a smaller volume of water is being heated, it comes to the boil much more quickly. As bubbles of vapour form, though, they don't shoot to the surface - they coalesce into a giant bubble that wobbles around within the liquid.

Without buoyancy, the vapour produced by boiling simply floats as a bubble inside the liquid after the heating has stopped. Surface tension effects cause the many small bubbles produced to coalesce into one large sphere.

Much of this could be predicted from existing theory, but to learn the fine details of the process and to look for unexpected behaviours, a real experiment was necessary.

"There were many fundamental issues that were still not understood well," says Dr. Herman Merte, the Principal Investigator for the experiments. Merte, who some see as a kind of "founding father" of microgravity pool boiling research, devised the experiments.