The Great Ketchup 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.

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."

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

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.