17 July 2012
Cornstarch and water have launched a thousand geeky pool parties. Stirred together in roughly equal proportions, they form a fluid that turns miraculously solid for a fraction of a second wherever it’s struck. This means, as numerous YouTube clips attest, that you can run across the surface of a wading pool filled with the gooey mix without sinking. As long as you keep up your speed, stepping stones sprout out of the fluid and bear your weight.
Why the stuff does this is a puzzle. Scientists have usually studied it by pouring a teaspoonful on a metal plate, sliding another plate across the surface, and recording how the fluid pushes back. But that presents a problem: when you run across a pool of the fluid, after all, you don’t slide on it, you stomp on it. Now the mystery–on which profound science admittedly does not turn but cool science definitely does–may at last have been solved by Scott Waitukaitis, a graduate student in physics at University of Chicago, whose work was just published in no less a venue than last week’s issue of Nature.
Waitukaitis began his research by talking the problem over with his adviser, physics professor Heinrich Jaeger, with whom he crunched the numbers and found that the theories based on the teaspoon experiments were nowhere near explaining how the fluid could support the weight of a human being. Mulling this, Waitukaitis devised his own experiment, which at first involved spending a lot of time throwing balls into buckets of cornstarch and water. After several disastrous attempts to mix a large batch of the material by hand–“It’s incredible,” he says, “how easy it is to get a shovel stuck in this and not be able to get it out.”–he started using a small cement mixer and, to protect his clothes, took to wearing a blue jumpsuit around the lab. His previous work went on hold as he developed more and more elaborate ways to measure the movement of the fluid, culminating in a series of experiments in which the tip of an aluminum rod, dropped from above, slammed into about 7 gallons of cornstarch and water. A battery of instruments then watched what happened.
The most basic of the detectors was a simple high-speed video camera, which watched where the rod hit the surface and recorded the shape of the depression it formed. An accelerometer in the rod told Waitukaitis how fast it was accelerating and decelerating, which, when coupled with the rod’s mass, let him calculate the force the fluid was exerting when hit. A force sensor at the bottom of the vat, just under where the rod struck, recorded how quickly the resulting stepping stone formed and hit bottom. An X-ray system watched the way tiny metal particles in the fluid moved in the milliseconds after the rod hit, providing a picture of how far the currents that had been produced would go.
After developing a mathematical model that incorporated all of these measurements and a few other parameters about the fluid, including the concentration of cornstarch grains, Waitukaitis and Jaeger developed a whole new theory to explain the stepping-stone phenomenon. The grains of cornstarch, they concluded, are so closely packed together that when the rod hits them, they have nowhere to go–like cars in a traffic jam. As with the traffic jam, untangling slowly is easy enough to do. But rushing things only cause you to rear-end or side-swipe the surrounding cars–or cornstarch grains. In both cases, this results in what is effectively a solid mass of material.
The phenomenon is not only dramatic, it’s almost instantaneous. If the foot of a running human smacks into the fluid at a speed of 3 feet per second (.9 m/s), a huge stepping stone, two feet (.6 m) thick, springs into being beneath the surface in less than 50 milliseconds, Waitukaitis estimates. “If you filled an ocean with this stuff, would you be able to run on it? With the original mechanisms, we’d say that you’d probably sink in,” Jaeger says. “Scott’s mechanism works.”
Not everyone agrees that that mechanism is totally new. One theory from the teaspoon experiments involves the cornstarch grains bunching together into clusters, and its author, John Brady of the California Institute of Technology, thinks that the process at work in the new experiments could simply be that theory scaled up hundreds of thousands of times. At this point, says physicist Eric Brown of University of California, Merced, who was not involved in the research, the most accurate explanation could be that the models are both true for different situations.
“The difficult issue is that it hasn’t been shown yet that they are connected,” Brown says, “because they’ve been developed for systems that are so far away from each other.”
In addition to providing an explanation for what happens when the fluid is struck, Waitukaitis and Jaeger have confirmed an intriguing phenomenon that does have practical implications–but only for cornstarch pool partiers. The stepping stone grows so quickly that if the pool is shallow, the solid will strike the bottom as you run. It will then rebound upwards and push back even harder against your foot. “The shallower the pool is, the greater the effect will be,” Waitukaitis says. So if you’re looking to run the 500-yard dash on cornstarch and water, less is more.
The Jaeger lab has yet to host such a pool party, in part because it turns out to be very difficult to dispose of wet cornstarch. Waitukaitis managed to destroy the plumbing of the lab downstairs by trying to wash it all down the sink. But for people nonetheless looking to get their own parties started, Waitukaitis has a piece of advice on how to begin: “I would definitely recommend a cement mixer.”