I love a good juice box as much as the next guy, but imagine if every single thing you drank, from a hot cup of coffee to a glass of wine at the end of the day, came out of a Capri Sun pouch. For astronauts aboard the International Space Station, this is business as usual – or at least it was. By repurposing the calculations used to create fuel tanks for rockets, NASA and research and development firm IRPI have created the first-ever cups that work in space.
Here on Earth, if you tipped a coffee cup upside down, the liquid would fall out because of gravity. That part is obvious, but what you might not realize is that other, weaker forces are simultaneously trying to keep the liquid in the cup. One such force is surface tension.
On the ISS, surface tension dominates the behavior of liquids, because there is little gravity to overwhelm it (as we’ve seen in commander Chris Hadfield’s viral demonstrations). In this environment, liquid doesn’t fall towards your mouth when tipped, because it’s attracted to the bottom and sides of the cup (adhesion), as well as to itself (cohesion). These physical properties could be viewed as a problem, but the research team used them to their favor by designing a cup that maximizes surface tension on one side, forcing the liquid within to “climb” the walls.
“The cross-section of this cup looks like an airplane wing,” explains astronaut Dr Don Pettit. “The sharp angle – that is, the narrow angle on the pinched side – will wick the liquid towards the lip. It’s the same concept used in fuel tanks for rockets, so they can re-ignite in micro gravity.”
Because the walls in the pinched segment of the cup are close together, the adhesion force between the liquid and cup is stronger than the cohesion force keeping the liquid together. “So as you sip, more fluid keeps coming up, and up, and up,” says Pettit. This capillary action is much like what happens when you place a paper towel over a puddle.
The cups have taken some heat on the web, namely by commenters claiming the venture to be wasteful. But the ability to refill cups from a common source could actually allow for less material waste on the ISS. “This very well may be what future space colonists use when they want to have a celebration toast,” says Pettit.
The next step will be to test the behavior of different liquids in the cup, which could help with future spacecraft design and safety.
“With this cup, most everything is taken care of passively by the shape of the cup,” says NASA. “There isn’t a straight line in it. There are no moving parts. Wouldn’t it be nice if all the fluid systems on spacecraft worked like that? We know it would result in less worry on the ground. The simpler things are, the more robust their function and the less time is needed for maintenance.”
IMAGES: NASA, IRPI