An-ti-ci-pa-tion

June 04, 2008

Good things sometimes come to those who wait, even if it is shadowed by tragedy -- and even if it takes five years. That's how long it took to recover data from an experiment on the doomed Space Shuttle Columbia, which burned up and disintegrated upon re-entry in 2003. Five years later, that recovered data is shedding light on why shaking certain types of complex fluids causes the material to flow faster. You know, like ketchup, nail polish, or paint: these fluids are normally thick and viscous, but become thin and flow like water whenever they're stirred or shaken. Physicists call this shear thinning, but they're not quite sure how or why it happens. For decades, the prevailing theory has been that the harder you stir, the thinner the complex fluid will become.

Viscosity is loosely defined as how much friction/resistance there is to flow in a given substance. Usually, a fluid's viscosity is largely dependent on temperature and pressure: the spirits that make up James Bond's martini will continue to flow regardless of whether it is being shaken or stirred. But in a complex fluid, like ketchup, the viscosity changes in response to an applied strain or shearing force, thereby straddling the boundary between liquid and solid behavior. (A similar effect can be seen in a popular high school experiment involving a mixture of corn starch and water, dubbed oobleck, that stiffens in response to an applied force.)

The thing is, it's tough to study something like ketchup in a controlled experiment because there's so many ingredients. Physicists need something closer to an idealized gas or liquid (technically, a gas is a fluid) under controllable conditions -- something like xenon, a noble gas whose molecules consist of a single atom. Of course, xenon is a simple fluid, and thus doesn't exhibit shear-thinning, at least under normal conditions. But put it on the space shuttle in orbital freefall, and adjust the pressure and temperature of the contained experiment to push the xenon to the critical point, and it should start to exhibit shear-thinning, just like complex fluids.

So that's what Robert Berg and his colleagues at the National Institute of Standards and Technology did: they designed an experiment to test the critical viscosity of xenon aboard Columbia -- an experiment that was cut short when the shuttle exploded, scattering debris for hundreds of miles across Texas and Louisiana.

NASA's recovery team managed to recover the computer hard drive with all the data among the debris. And amazingly, the original cell that housed the experiment was also recovered, its xenon atoms intact, even though the outermost of the concentric shells in which the package was wrapped had burned away during re-entry. Berg and Company have now analyzed the recovered data, and concluded that stirring the xenon "fluid" (via a tiny mesh tennis racket) harder decreased the viscosity, just like what happens in more complex fluids like ketchup and paint.

It's a small advance in the grand scheme of things, to be sure, but that's how science progresses: in increments that gradually add up over time. Maybe one day, we'll have a sufficiently complete theory of this phenomenon that we can anticipate sudden changes in viscosity -- before we accidentally douse our burger and fries with an avalanche of ketchup.

Photo: "Xenon," from About.com's online Gallery of the Periodic Table of the Elements.