Scientists have a pretty good idea what happens to liquids on Earth when they are poured, sloshed or spilled.
But deep space is another matter altogether.
NASA engineers spend a surprisingly amount of time trying to understand how liquids behave in zero gravity. It isn't so much out of curiosity as much as a concern that its fuel tanks are safe.
NASA's most powerful rockets use liquid fuel, and when these rockets blast off, the propellants slosh around.
Most of the models out there can predict what happens to fuel on Earth but until now, not in space. Surface tension and capillary effects guide weightless propellants. Far from Earth, they could slosh and froth in unexpected ways.
"Modern computer models try to predict how liquid moves inside a propellant tank," said Brandon Marsell of NASA's Fluid Group at the Kennedy Space Center. "Most of the models we have were validated under 1G conditions on Earth. None have been validated in microgravity."
Enter the SPHERES-Slosh experiment.
"The International Space Station provides the perfect environment to conduct liquid behavior studies in microgravity," said the principal investigator, Paul Schallhorn, also at Kennedy. "So we have designed an experiment that simulates how rocket fuels move around inside their tanks."
Built by Prof. Dan Kirk and colleagues at the Florida Institute of Technology, SPHERES-Slosh is a fluid chamber buffered on each side by a pair of bowling ball-sized robots. Together, the robots move the chamber back and forth to mimic common spacecraft maneuvers such as the "BBQ roll," so-called attitude adjustments, and engine shutdowns.
In 2014, astronauts supervised the robots as they made three test runs using chambers 20 percent and 40 percent filled with fluid - much like a partially spent fuel tank.
The experiments are carried out using water mixed with a bit of green food coloring. A safe fluid for the space station, it also acts similarly to hydrazine, which is a propellant used by satellites.
During the experiments, which can last as long as six hours, cameras, gyros and accelerometers record the motions of the water.
"We are getting great data," Marsell said. "So far, our computer models on Earth have done a good job predicting wave motions inside the chamber."
But the results have offered up one big surprise.
"It has to do with bubbles," Schallhorn said. "The way bubbles form and interact inside the chamber is surprising - and not predicted by our models. We were taken off guard with what we saw in the data."