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April
25, 2008: Let's do a little science experiment. If
you have a can of whipped cream in the fridge, go get it out.
Spray a generous dollop into a spoon and watch carefully.
 Notice
anything interesting? The whipped cream just did something
rather puzzling. First it flowed smoothly out of the nozzle
like a liquid would, and then, a moment later, it perched
rigidly in the spoon as if it were solid. What made it
change?
(While
you're pondering this question, insert spoon into mouth, in
the name of science.)
Whipped
cream performs this rapid changing act because of a phenomenon
called "shear thinning." When part of the foam is
forced to slide or "shear" past the rest of the
foam, the foam "thins." It becomes less like honey
and more like water, allowing it to flow easily until the
shearing stops.
Shear
thinning occurs in many substances--e.g., ketchup, blood,
motor oil, paint, liquid polymers such as molten plastic--and
it is often crucial to how a substance is used. For instance,
excessive shear thinning of motor oil is unwanted because
it reduces the oil's ability to protect engines from wear,
while shear thinning of paint allows it to flow smoothly from
the brush but stay put on the wall. It also allows ketchup
to flow from the bottle but not drip off your french fries.
Yet,
for years, scientists have asked themselves the same question
you just did: What made it change? The inner workings
of shear thinning are not fully understood.
"Details
depend on interactions in the fluid at the molecular level
and those interactions can be devilishly complex," says
fluid physicist Robert Berg of the National Institute of Standards
and Technology. "Even for very simple fluids, fundamental
theories have never been directly verified."
Until
now. The first real-world confirmation of a theory for how
shear thinning works in a simple fluid has come from an experiment
that flew aboard the final flight of Space Shuttle Columbia.
"We
showed that a leading theory is basically correct, "
says Greg Zimmerli, Project Scientist for the experiment at
NASA's Glenn Research Center. "This is an important step,"
adds Berg, the experiment's principal investigator.
Most
of the data from the experiment, called Critical Viscosity
of Xenon-2 (CVX-2), was beamed down to scientists on the ground
before the shuttle's destruction during reentry into Earth's
atmosphere. Remarkably, the hard drive from the experiment
survived the disaster and was found amid the wreckage, and
technicians were able to recover the rest of the data.
Above:
The thumbnail-sized patch of "window screen" suspended
between the electrodes is the paddle that stirred the CVX-2
xenon sample.
CVX-2
was designed to study shear thinning in xenon, a substance
used in lamps and ion rocket engines. Xenon is chemically
inert, so its molecules consist of a single atom -- it's about
as close as you can get to the flying billiard balls of an
idealized
gas or liquid. Unlike whipped cream, which is made of
long, complicated organic molecules, xenon would be relatively
easy to understand.
"It's
a simpler fluid for the theorists to try to grasp," Zimmerli
says.
Simple
liquids like xenon don't normally experience shear thinning.
They're either thick or thin, and they stay that way. But
this changes near the "critical point" -- a special
combination of temperature and pressure where fluids can exist
as both a liquid and a gas simultaneously. At their critical
point, simple fluids are able to "shear-thin" (a
verb) just like whipped cream does.
Below:
The phase diagram for xenon illustrates the concept of critical
point -- the highest temperature and pressure at which xenon
can exist as a liquid and a gas at the same time.
 Xenon
at the critical point resembles a hazy fog, a slurry of microscopic
pockets of slightly higher or lower density. These tiny regions
of varying density are constantly appearing and disappearing
in a seething froth, giving the pure xenon some of the structural
complexity of mixtures like blood.
CVX-2
had to be done in space: Critical-point fluids are easily
compressed. On Earth they collapse under their own weight
and become denser at the bottom. In orbital free-fall those
differences vanish -- a key requirement for a good experiment.
To
test shear thinning, CVX-2 adjusted the temperature and pressure
in a small cylinder to bring xenon to its critical point,
and then gently stirred the fluid with a nickel-screen paddle.
By measuring how strongly the fluid resisted the movement
of this paddle, the experiment could determine the xenon's
thickness. CVX-2 searched for changes in this thickness as
it slowly changed the speed of the stirring and the temperature
of the fluid.
Results
nicely matched the predictions of dynamic mode-coupling theory.
"This more fundamental understanding could help us build
better theories for shear thinning in fluids more complex
than xenon," Zimmerli says.
That
would be good news for, say, engineers who want to design
high-performance oils for automobiles or manufacturers who
would like to create liquid plastics with just the right shear
thinning properties for a particular mold. The sky's the limit.
Whether
it would be possible to improve whipped cream, however, is
highly debatable.
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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