(for other entries in the Chemistry in Space series, click here)
Who knew boiling a liquid was so complicated? When you put a pot of water on the stove or heat your reaction-in-toluene solution in an oil bath several things happen. The liquid closest to the heating element starts to get hot. Convection circulates the hot liquid up and the cold liquid down due to the density differences of hot and cold liquids. Eventually, the liquid near the heating element becomes hot enough to move into the vapor phase and bubbles start to form. Buoyancy causes the bubbles to float to the surface and pop, while more convection continues to circulate the water. Eventually, you get a rolling boil.
Everything changes in the microgravity environment of space. Buoyancy and convection no longer play a role. The heated fluids no longer circulate and the bubbles no longer naturally rise to the surface. So what happens when you try to heat a liquid to boil in microgravity? Astronauts tested this during the course of several space shuttle missions during the 1990s. They arrived at some very interesting conclusions.
First, the liquid nearest the heating element starts to get hot, just as it does on Earth. But it doesn’t rise and circulate due to convection. It just gets hotter and stays next to the heating element. It eventually gets hot enough to move into the vapor phase, just as it does on Earth, but the bubbles don’t rise to the surface and pop. Instead, they stay next to the heating element and coalesce into one giant bubble. Eventually, the size of the bubble becomes larger than the heating element and there is no longer any liquid in contact with the heating element. This insulates the liquid from the heating element and leads to a “dry out” where there is no more boiling and the temperature of the heating element “begins to soar.”
(click on the image to go to the NASA page describing Zero G Boiling and to see an awesome movie of boiling in action)
All of this is predicted by theory, but it’s nice to have the chance to do some of those proof of principle experiments for the first time ever. It reminds me of what some of the pioneers of science must have felt when working out some of the fundamental theories of chemistry and physics that we don’t even realize we take for granted today.
An interesting variation of this experiment was conducted impromptu by an astronaut on the International Space Station in 2003. Don Pettit* was performing repair operations using a soldering iron. He decided to put a few milliliters of water on the hot surface. The water droplet formed a blob around the soldering iron and kinda wobbled there. As expected, the water heated up and began to boil. Surprisingly, though, this time the boiling looked much similar to boiling on Earth.
My working theory is the small amount of water and the inherent jostling of the system (the soldering iron looks like it was held by hand in front of the camera) caused enough motion in the water to move the bubbles around. The bubbles could bump into each other and coalesce. The size of the bubbles quickly reached the surface (unlike the bulk boiling experiment described above) and were allowed to pop. Thus, it is by accident, in my opinion, that the boiling looks like it would on Earth. It’d be interesting to repeat the experiment with the soldering iron held steady by vice grips or something.
(click on the image to go to the NASA page on the soldering iron boiling experiment and to see an awesome movie of this microgravity microboiling in action)
*Also inventor of the super awesome zero-g coffee mug.