The Great Ketchup Mystery

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.

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.

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.