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Any child who has played outside in the rain can describe how raindrops seem to skip along the sidewalk, then become part of a puddle or stick to the ground. However it would surprise most children to learn that traveling water droplets behave quite differently under zero gravity conditions, making coalescence to puddles or adhesion to surfaces more difficult phenomena to achieve! This difference in behavior is due to films of gas that develop between a water droplet and a surface it approaches. This layer of gas, called a squeeze film, prevents or delays contact between water and the surface from occurring. Spinning drops can also entrain gas films that form an even more stable boundary. With enough time, this gas layer can be squeezed away or disturbed enough to allow contact between the droplet and surface. The droplet then adheres in order to reduce its surface energy. In the absence of gravity, the contact time is greatly reduced, impeding coalescence of water and the wetting of impacted surfaces. Here is a video of water droplets inside a bubble in space. This was one of Don Pettit's zero-G water experiments done while he was on a space mission. space mission. Notice how the droplets bounce on contact and only merge occasionally.
On the Expedition 6 space shuttle, Don Pettit documented this difference. He injected an air bubble into a large sphere of water and then injected small drops of water inside the air bubble. The droplets bounced around the air bubble, contacting each other or the surrounding water about eight times before coalescing (Pettit)! Though this difference between water behavior on earth and under zero gravity has been noted, little qualitative analysis has been done. For example, little is known on the effect the contact surfaces has on if/when an approaching water droplet will adhere to it under zero gravity conditions. See other NASA water experiments below: http://microgravity.grc.nasa.gov/balloon/mpeg/balloo01.mpg |
