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IMAGE: A demonstration of surface tension of water in microgravity
Using a piece of safety wire, ISS Science Officer Don Pettit demonstrates the surface tension of water in microgravity.
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Don Pettit Space Chronicles

Expedition Six
Space Chronicles #5

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By: ISS Science Officer Don Pettit

Water Observations

One time I sent a marble-sized drop of soap-water from my shower off on its own trajectory. It was filled with tiny bubbles. Like a little planet, the drop was spinning on its axis. I observed that all the bubbles quickly moved to the center along a line about the axis of rotation. Obviously, the rotation creates a radial acceleration and the resultant forces move the bubbles to the center. This was behaving just like I had learned in my physics classes. The rotating linear clump of bubbles presented itself like a tiny tornado. Theover-all visual effect, with a hazy bubble core surrounded by clear water reminded me of a cat-eyed marble.

The next question of course, was what would happen if the bubbles were replaced with particles of density greater than water? It took me a few days to find a collection of such particles, and when I did, it was sweet. I took granulated sugar and tea leaves from one of our bags of tea and placed them in a drop of water. To do this feat without having to juggle dozens of little grains and potentially releasing them into the air, I first injected cold water into the tea pouch. I used cold water to slow down the rate at which the sugar will dissolve. Then I cut it open along one edge and gently spread the plastic apart, creating a passage into the bag.

The water mixture formed a well-behaved concave surface that tightly clung to the inside. You can now reach into the bag with a pair of scissors and cut open the teabag, releasing the tea leaves into the water. Then I took a 50 ml irrigation syringe and sucked out about 40 ml of the water suspension that included sugar grains and tea leaves. A few big gulps disposed of the excess liquid in the bag. I figured it would make tea in my stomach. The tea in the syringe looked like one of those shake-and-watch snow scenes that sit on people's desks, except this one never quits snowing. I carefully made about a 20 mm diameter drop of this mixture that clung to the syringe tip due to the near 90-degree contact-wetting angle of water on polyethylene. Using a thin Teflon canella as a pusher, I carefully coaxed the water blob off the syringe tip by passing the canella several times through the water. Since Teflon has a near 180-degree contact-wetting angle for almost any liquid, it passes through the water without dividing the drop much like a magician's saw can pass through the bikini-clad lady without cutting her in half. A small force is imparted into the water with each pass that eventually frees it from the syringe tip.

Like a caterpillar that emerges from its chrysalis becoming a beautiful butterfly, this curvilinear blob of water once freed of the syringe tip takes on the form of a most beautiful sphere. It glistens as if looking into the eyes of an inquisitive child. A small rotation is imparted by passing the Teflon canella through the edge tangent to the sphere's center of gravity. Upon imparting a rotation, a most wonderful sight was seen. A few trapped bubbles went to the center axis of rotation as expected. The sugar crystals, slow to dissolve, went to the perimeter and seemed to bounce along the inside meniscus like some alien form of water skipper that scoots under the surface instead of above the surface. The dissolving sugar creates refractive index gradients that make it possible to see density plumes swirling towards the core.

All this was pretty much textbook physics. The tea leaves though, continued to circulate around and around on the surface forming patterns that reminded me of continents undergoing some form of plate tectonics in miniature. What I was observing was not in any of my physics classes, which of course, is science at its best.


Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 05/14/2003
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