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NASA Science News for February 25, 2003

Saturday Morning Science
========================

Even in their spare time, astronauts onboard the ISS are discovering
new and surprising things about nature. 

February 25, 2003: Space station science officer Don Pettit always
looks forward to Saturday mornings. 

Like the other members of the International Space Station's 3-person
crew, he's busy most of the week doing research and building the ISS,
where he's been living for the past three months. "Saturday is when we 
have a bit of free time," Pettit says. Some of the crew read books, 
play musical instruments or watch movies. "I prefer to do 'Saturday 
Morning Science'--fun experiments of my own design."

A few Saturdays ago, he had his heart set on bubbles. "We have a copy
of C. V. Boys' book Soap Bubbles here on the ISS. It was published in
1911 and it's still a wonderful treatise on thin films. Every space
station should have a copy," he laughs. "I wanted to see what thin
films and bubbles might do in zero-g and felt it was a topic ripe for
discovery." 

Pettit prepared a solution of water, soap, and glycerin, and fashioned 
a bubble-wand from thin wire--a loop that could be re-sized from 3.5 
cm (about 1.5 inches) to more than 15 cm (6 inches) in diameter. The 
experiment was ready. "But first," recalls Petit, "I decided to try a 
'dry run' with water only, no soap." 

He inserted the wand into a zero-g beaker and pulled it out again.
"To my amazement," he says, "when the 2-inch loop was withdrawn, a
thin film of water clung tenaciously to the loop. I've never before
witnessed such a large-scale film of water." 

To fully appreciate Pettit's discovery, just try the experiment in
your own kitchen (on Earth). Fill a bowl with drinking water and
fashion an adjustable loop of wire. No matter how hard you try, it's
impossible to make water stretch across a loop wider than about 1 cm
(0.4 inches). Any film you do make, furthermore, will be fragile. A
gentle bump or breath of air will cause it to burst. 

Pettit's films, on the other hand, were 5 to 11 cm (2 to 4 inches) in
diameter and remarkably sturdy. He could shake them vigorously, blow
on them ... even paint on them. "They were like little sheets of
rubber," he marveled. "They could withstand all sorts of mechanical
torture." 

Why are space films so tough? To understand the difference between
Earth films and space films, it's helpful to understand surface
tension. 

Surface tension is what allows water bugs to scamper across ponds
without sinking. It's a stretchy "skin" across any wet surface that
resists penetration. This skin exists because water molecules are
electrically charged. The positive end of one molecule is drawn to the 
negative end of another: water literally clings to itself! 

The electrical attraction between water molecules, and thus the
surface tension of water, is the same on Earth and in space. There's
no difference. 

What is different is the competition between surface tension and
gravity.

Let's say you're on Earth and you have a thin film of water held
parallel to the ground. The pull of gravity causes the film to sag in
the middle. Water drains downward from the edge of the loop; a little
pool forms. The film sags more. The pool gets bigger and bigger until
its weight rips the film apart. 

If you're in Earth orbit, however, the film would be in free fall--
weightless. The film doesn't sag. The central pool never forms.
Surface tension therefore wins the competition with gravity, and the
result is a sturdy long-lasting membrane. "Some of our films lasted
longer than 12 hours," notes Pettit. 

What do you do with a membrane of water? The entertainment value alone 
is impressive. 

Pettit spent some time simply making the wand bigger and smaller.
"Oscillating the loop with a period of about 2 seconds distorted the
film with remarkable patterns like you might see in a rubber membrane
driven by a sound oscillator. The back-and-forth displacement at the
center was several centimeters." 

Next, Pettit injected some tiny mica flakes into the film. This
allowed him to observe otherwise-hidden flows and swirls. "I blew on
the film using my own breath," says Pettit, and fascinating patterns
emerged--some that looked like spiral galaxies. "These tracer particle 
patterns lasted for well over four hours." 

Then it was time to paint. On one film, Pettit deposited four drops of 
food coloring: red, blue, green and yellow. Using a syringe with a
thin tip (a canella), he shot a stream of air across the watery
canvas, pushing the colors to and fro. One of his paintings looked
like an eagle, others like abstract art. 

"I wonder what someone like Matisse could do with this ephemeral
medium?" wonders Pettit. "Eventually, all the colors blended together
yielding a rather dull looking green. I think we've discovered the
true color of the Universe!" he joked. 

Seriously, though, these films highlight the value of space for
fundamental research in fluid physics. Fluid flows on Earth are
complicated by gravity-driven convection and 3D motions. A 2D film of
weightless water is a splendid research tool that could yield data of
value to many industries on Earth. 

Meanwhile, Pettit hasn't forgotten his original experiment: "We never
got to the soap solution because we were diverted by plain water.
We'll do soap bubbles on another Saturday," he says.

What will they reveal? No one knows.

"Observations of nature, no matter how seemingly arcane, are like
peeling off one more layer from the great onion of knowledge, tickling 
your imagination with what you have found but always revealing yet 
another tantalizing layer underneath," says Pettit. 

"I hope we never get to the core."

Editor's note: Science{at}NASA will publish a series of stories about Don 
Pettit's Saturday Morning Science activities in the weeks ahead.  Stay 
tuned! And while Don's experiments are done for fun, motivated by his 
own curiosity, there is also serious scientific research underway 
involving fluids and foams in microgravity. We'll report on those 
experiments in an upcoming story.

Credits & Contacts
Author: Dr. Tony Phillips
Responsible NASA official: Ron Koczor 
Production Editor: Dr. Tony Phillips 
Curator: Bryan Walls 
Media Relations: Steve Roy

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