Phil Plait Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!

Recently, space station astronaut Reid Wiseman fell back to Earth riding a trail of fire as his Soyuz capsule rammed through the air. But shortly before he returned to the ground, he and his crewmates took some time while in space to play with … water.

In the weightless conditions of orbit, water behaves differently than it does on Earth. With the space station and all inside falling freely around our planet, other forces dominate the water’s actions, which provides for some eerie, cool, and very fun videos.




Weird, isn’t it? The two drops of water are floating, and for a moment after they touch nothing happens, until they suddenly merge into one, bigger blob.

They resist merging due to surface tension. I describe this in detail in an earlier post about water in space, but in short, the water molecules on the surface of a drop feel a weak electromagnetic force inward, toward the center of the drop, because water molecules are slightly asymmetric. The outside of the drop has an overall (though faint) positive charge. When two drops touch, that positive charge on each initially resists the other drop, like the north poles of two magnets repelling each other. But if they’re moving at the right speed, their own momentum overcomes that weak resistance. The water molecules in the drops rearrange themselves, and you get one bigger blob.

I love how the resulting blob moves! The water flows around inside due to the momentum of the collision, which settles down after a few seconds as surface tension and friction take over.




However, not every collision results in a merger. If you set up things just right, the repulsion between drops wins:

What I like most about that video—besides the droplets splattering around the bigger drop if their velocity is too low to overcome surface tension—is what looks like a second droplet inside the big drop. But that’s not water. It’s air.

I’ll let Expedition 31 astronaut André Kuipers demonstrate:

Photo by NASA/ESA

Kuipers injected a bubble of air into that drop of water, and without gravity acting on it, the air is free to move around inside the drop. The water in the drop acts like a lens, flipping the image of his face over, and the air bubble does the same thing, so inside that we see him right-side up. Neither the bubble nor the drop is perfectly symmetric, so the images are distorted.

I find this kind of stuff fascinating. I mean, we know how the science works, but to see it in action is delightful (and it’s not always obvious what will happen even when you know the science). The astronauts were clearly having a great time filming this; here’s a video of them playing with the water (note: this was recorded using a 3-D camera; you’ll need red/green glasses to see it at its best, and I do recommend watching it that way!):