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Cornstarch Physics Is Shear Nonsense

Liquid or solid? An aluminum bar bounces off a liquid-like cornstarch suspension.
Credit: Scott Waitukaitis, 2011

Filling a small swimming pool with cornstarch and water has long been a physicist's party trick. Step onto it slowly and you'll sink but run across quickly and the oozy mixture will support your weight—almost as though it has turned from liquid to solid. Several reasons have been offered for the phenomenon, but now researchers believe they have the real answer.

Mixtures of cornstarch, water, and other suspensions have been known as "shear-thickening" materials. Shear is the type of stress that exists when particles slide over one another, and scientists thought that if the shear stress in a cornstarch suspension exceeded a certain threshold, the thickness or viscosity would increase massively—enough to support a person's weight.

There are two ways this could happen. One involves the water between the cornstarch grains: At lower grain densities the water acts as a lubricant, but with enough shear the water suddenly doesn't have the time or space to get out of the way and the suspension locks. The other way considers the movement of the cornstarch grains themselves. With too much shear, the grains can no longer rise and fall over one another fast enough and, again, the suspension locks.

Except it's not really about shear, according to physicists Scott Waitukaitis and Heinrich Jaeger of the University of Chicago in Illinois. In a simple calculation, the researchers figured that the swimming pool trick would need at least 10 times more shear stress than is realistically achievable from a person running. And that's not surprising because when you run over cornstarch and water you're not really forcing the cornstarch grains over one another—you're compressing them.

A different solution was needed. "We decided, what better way to understand how these suspensions respond to impact than to shoot stuff into them?" says Waitukaitis.

So the researchers plunged a 370-gram aluminum rod from a slingshot at around 1 meter per second into a cornstarch suspension. They then observed the result with various instruments, including a high-speed video camera to measure the rod's acceleration, and x-ray imagers and force sensors to observe the suspension's movement. Waitukaitis and Jaeger discovered that, upon impact, the rod squeezed the cornstarch grains so that they jammed into a rigid core, which exerted an upward force sufficient to prevent the rod from sinking. The results are published online today in Nature.

The key to the core is its ability to grow very large very quickly. If the density of cornstarch grains is just 10% less than the density required for jamming, then the suspension needs to be quickly compressed by only 10% to jam. But that also means a compression of, say, 5 centimeters will jam the grains, and form a rigid core, to a depth of 50 centimeters. It is this sudden formation of a thick steppingstone that means you can run across a cornstarch suspension without fear of sinking.

Physicist Itai Cohen at Cornell University believes that this newly discovered mechanism doesn't discredit the previous mechanisms based on shear thickening, which have been tested in other experimental situations. "What these guys are saying is that in order to understand something like walking on a pool of cornstarch, there's a different effect you have to take into account."

Waitukaitis agrees. "It's not to say what the other folks have figured out is wrong," he says. "Those findings still apply to shear situations."

Even so, physicists still have plenty of investigating to do, partly to explain other aspects of the phenomenon, such as how fast one has to push to create the rigid core. That won't be for additional poolside antics—the military has a serious interest in creating materials based on cornstarch-like suspensions to protect soldiers and other high-risk personnel from impacts, while being otherwise flexible. Engineer Norman Wagner at the University of Delaware, Newark, has worked on such "smart" materials, and believes the latest knowledge will help in their development.

That doesn't stop the rest of us from having fun, of course. Waitukaitis reckons the ideal suspension can be made with 1.267 kilograms of cornstarch for every 1 kilogram of water, but you need to watch out for its ability to suddenly thicken. "Don't get discouraged if you find it hard to mix," he says.