We take boiling for granted. Heat some water up to 100°C (212°F) and bubbles appear as the water turns into a gas. However, you probably know that boiling water on a mountain happens quicker and at lower temperatures. This is because the atmospheric pressure up there is lower and bubbles can form more easily.
So what’s boiling water like aboard the International Space Station (ISS)? As it turns out, due to a serious quirk of physics, it’s actually quite difficult, as astronauts have discovered over the past few years.
Trying to boil water moving through a heat pipe – a component used to cool equipment aboard spacecraft – they found that even when the water was 160 degrees above its normal boiling point, the microgravity of the ISS kept it in its liquid form.
For safety reasons, the astronauts couldn’t actually “force” the boiling to occur by increasing the temperature any further. A liquid remaining as such above its boiling point is known as a “superheated” fluid. If suddenly exposed to lower pressures or the temperature increase allows bubbles to suddenly be able to form, a sizable explosion can occur.
Rather remarkably, the team of NASA and Rensselaer Polytechnic Institute researchers aren’t entirely sure why the water’s refusing to boil up there – but they do have a rather intriguing hypothesis.
According to their paper in Physical Review Letters, this lack of space-based boiling may be linked to the so-called “Marangoni effect”, named after the physicist that wrote his doctoral thesis on it back in 1865.
Pour yourself a glass of wine and hold the glass up to some light. You’ll notice what look like teardrops above the pool of wine clinging to the glass. Known as “tears of glass”, this is all down to something called surface tension.
The tears of wine effect is more pronounced when the alcohol content of wine is higher. FlagSteward/Wikimedia Commons; CC BY-SA 3.0
Water, thanks to its strong hydrogen bonds, has a high surface tension, so it’s able to cling on to things like glass more than alcohol is able to. So those tears you see on the wine glass are water films that have separated from the alcohol.
These heat pipes also contain these “tears”. One end of the pipe is hot, where evaporation is expected to occur, and one is cold, where condensation should take place. Water in both sections have a high surface tension, but the cold water section has an even higher surface tension than the hot water section.
The Marangoni effect dictates that the cold water drags the hotter water towards it, which allows the superheated water to condense. This can happen on Earth, but thanks to the weak gravity aboard the ISS, the water isn’t stuck down inside the pipe and it can flow a lot more smoothly.