Shear Mystery

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.

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"Shake and shake the ketchup bottle. None'll come, and then a lot'll." --Richard Armour

June 7, 2002:  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.

Above:  The sudden surge of ketchup from a bottle typifies an important and puzzling property of many liquids: shear thinning. Credit: MacKingShow.com.

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.

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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 phenomemon 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."

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."

Below: When a fluid is being sheared, as in the right diagram, some parts of it are moving faster than others. Image courtesy NASA and NIST. Click on image for more information; Adobe Acrobat reader required.

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.

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.

Right: 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.

Gregory Zimmerli, a scientist at NASA's Glenn Research Center, 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.

Below: The patch of "window screen" suspended between the electrodes is the paddle that will stir the CVX-2 xenon sample.

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 behavior 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 behavior," 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.

Space Research and You -- an overview of the STS-107 shuttle mission from NASA's Office of Biological and Physical Research.

What is shear thinning? -- a Q&A from the Argonne National Laboratory

Experiment home page -- detailed information about this experiment, including contact information for the researchers

Above:  Photographs of carbon dioxide in a bulging glass container illustrate the "hazy fog" at the critical point. As the container is cooled from left to right, the carbon dioxide changes from a clear, dense gas, to a hazy fluid at the critical point (second image from left), and finally to dense liquid carbon dioxide beneath dilute carbon dioxide gas (far right). On the right, three glass bobs float at the boundary between the liquid and the gas. Reproduced with permission, Chemical & Engineering News. Copyright 1968, American Chemical Society.

Learn more from the web site "Carbon Dioxide at its Critical State" by Stephen Lower of the Dept. of Chemistry at Simon Fraser University.

Xenon -- information on the element used in this experiment, from WebElements.com

National Institute of Standards and Technology -- home page

Fact sheet -- for the experiment discussed in this article, from NASA Office of Biological & Physical Research