a piece of safety wire, ISS Science Officer Don Pettit
demonstrates the surface tension of water in microgravity.
Space Chronicles #5
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ISS Science Officer Don Pettit
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
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
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.