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Some fluids have a mysterious property: one
moment they're thick, the next they're thin. Physicists aim to find
out why with the aid of an experiment in space.
by Patrick L Barry and Dr Tony
Phillips
Everyone has fallen prey to the ketchup
bottle at one time or another.
After struggling to dislodge a meager few drops of the red liquid,
an avalanche suddenly gushes out and buries your perfectly cooked
burger. With suspiciously perfect timing, the ketchup changes from
a thick paste to a runny liquid.
If you find yourself splattered and wondering "why?",
you're in good company. Theoretical physicists are puzzled, too.
Ketchup is one of many complex fluids - including
whipped cream, blood, film emulsions, nail polish and some plastics
- that share a property called "shear thinning." Normally thick
like honey, they can become thin and flow like water when stirred
or shaken.
Paint is another example. How can paint be thin
enough at one moment to flow from a stroked brush, and an instant
later be thick enough not to drip down the wall? Shear thinning
again.
The phenomenon is common enough, yet scientists
aren't sure why it happens. Says researcher Robert Berg of the National
Institute of Standards and Technology, "the details depend on interactions
at the molecular level [in the fluid], and that is still poorly
understood."
The sudden
surge of ketchup from a bottle typifies an important and puzzling
property of many liquids: shear thinning.
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Current theories cannot predict the
thickness (or "viscosity") of many fluids. It's a problem that vexes
physicists and manufacturers alike. Suppose, for instance, that
a plastics-maker needs to know how some new polymer "goop" might
flow through a pipe. The only way to find out might be to try it
- a tedious and sometimes innovation-stifling process. What they
really need is a theory that works, a way to anticipate changes
in viscosity "before the ketchup explodes from the bottle."
Researchers hope that a space-experiment
called CVX-2 (short for "Critical Viscosity of Xenon-2") will soon
provide new data about the basic physics of such fluids. Berg is
the principal investigator for the experiment, which is slated to
fly this summer onboard space shuttle Columbia (STS-107).
CVX-2 is 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 ketchup, which contains many
ingredients ranging from microscopic ions of dissolved salt to visible
chunks of pureed tomato, xenon should be relatively easy to understand.
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.
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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 should be able to "shear-thin" (a verb) just like
ketchup does.
Gregory Zimmerli, a scientist at NASA's Glenn Research
Centre, explains that "fluids at the critical point resemble a hazy
fog - a flurry of little regions constantly fluctuating between
liquid-like and gas-like densities. Theory predicts that this fine-grained
structure should make the simple fluid shear-thin, like more complex
fluids do." (Zimmerli is the project scientist for the CVX-2 experiment.)
When CVX-2 reaches orbit, researchers will adjust
the pressure and temperature of a xenon sample within the test chamber
until it reaches its critical point. A tiny vibrating paddle will
then stir the xenon and, if all goes as planned, cause it to thin.
The patch
of "window screen" suspended between the electrodes is the
paddle that will stir the CVX-2 xenon sample.
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Why do the experiment 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 density differences vanish - a key requirement for a good
experiment.
Researchers will probe the physics
of shear thinning by varying the temperature of the xenon and amount
of stirring it receives. The same paddle that stirs the sample will
also measure its viscosity, just as you might estimate the thickness
of honey by trying to move a spoon through it.
At least that's what scientists are
hoping will happen.
The ketchup-like behaviour of pure
fluids at their critical point is still only theoretical. Even simulations
using supercomputers can't prove the theory. "Especially near the
critical point, there aren't computers that can simulate the fluid's
behaviour," notes Berg. "The chains of interactions between molecules
are so long that computers just aren't powerful enough to do it."
Consider that the next time you whack
the bottom of a ketchup bottle. Even supercomputers can't predict
the outcome.
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