If water has no shape, how come in space it always becomes a sphere?
If water has no shape, how come in space it always becomes a sphere? asks Raquel Allen, a student in Brookville, NY.
At breakfast on the International Space Station, a splotch of orange juice can land on your shirt–even if it was spilled by someone across the room. That’s because in the microgravity of the space station, spilled liquids collect into round, floating drops.
Liquids, content to wait at the bottom of a glass on Earth, behave very differently in the near-weightless conditions found in the orbiting space station or shuttle. According to NASA scientists, the pull of Earth’s gravity on the space station and its occupants is substantial: about 90 percent of the force at the Earth’s surface. But since the space station is continuously falling around our planet, the astronauts and objects on board are in a kind of free-fall, too, and feel nearly weightless.
So what makes liquid water ball up in microgravity? Molecules in a parcel of liquid water are mutually attracted, but can slip and slide past each other. That’s how liquid water, unlike solid ice, can take on the shape of the container it’s poured into, such as a drinking glass.
But water molecules at the surface aren’t much attracted to gas molecules whizzing in the air above them. Their main attraction is downward and sideways to other water molecules in the glass. The result is a tense, tight surface–almost like a thin, rubbery skin on the water. (Some insects can walk across this “skin” on the surface of a pond.)
This surface tension is the key to the shape of liquid water spilled in microgravity. Water is free to leave an open container in microgravity, since gravity isn’t keeping it pinned to the bottom. As a parcel of water free-falls in the space station, surface tension pulls the water into a sphere. How come? Since the parcel is free-floating blob, it has one smooth surface exposed on all sides. All molecules on the surface tend to be tugged down and sideways with equal tension by their fellow molecules. And so the blob of water pulls into a compact sphere — the most efficient shape in nature, with the smallest possible surface area.
(Watch a water balloon burst in microgravity at www.space-video.info/misc/balloon.html.)
We can see the surface tension effect on Earth each time it rains. Water free-falls from clouds as drops, each held in its own “bag” created by surface tension. The tear-shaped raindrops would be round spheres if it weren’t for the drag of the air they fall through.
Scientist and astronaut Don Pettit worked on the space station for five months in 2003. In his “Saturday Morning Science” experiments, he was often astonished by the behavior of liquid water in near-weightlessness. Watch Pettit’s experiments with water spheres (including inserting a tablet of Alka-Seltzer) at www.freesciencelectures.com/video/waves-bubbles-and-reactions-in-a-free-sphere-of-water. Bored with drinking tea the old-fashioned way? Watch Pettit consume tea blobs with chopsticks at http://science.nasa.gov/ppod/y2003/07apr_hightea.htm. Many more videos of Petit’s adventures with weightless water can be found on Youtube.
