Scientists Reveal Why You Can Walk on Water When It's Combined with Cornstarch

By Chelsea Whyte on July 13, 2012 10:05 PM EDT

cornstarch
Non-newtonian liquids, like cornstarch suspended in water, can act like solids when hit with force, and now scientists know why. (Photo: Creative Commons: mac.rj)

Many kitchen sink scientists have marveled at the strange properties of cornstarch. When combined with water, the powdery substance makes a milky liquid when left alone. But slap the liquid goo hard and your hand will hit what feels like solid material.

So how does cornstarch in water do its two-faced trick? Scientists at the University of Chicago studied this non-Newtonian liquid - one that can instantaneously turn into a solid under the force of a sudden impact - and their findings, published in the journal Nature, explain a natural phenomenon that has been stumping scientists since the 1930s.

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Scott Waitukaitis and Heinrich Jaeger found that compression on the surface of the liquid can generate a rapidly growing, solid-like mass in a suspension - or a liquid with micron-sized particles floating around in it.

"The corn starch grains are like tiny little rocks bobbing around in the water, very densely packed but not so densely that they're touching each other," study co-author Waitukaitis told BBC News. "We found that when you hit the suspension, a solid-like column grows below the impact site," he said.

To test compression in the suspension, they set up a large vat of cornstarch and water and drove a rod into the mixture, creating a shock-like moving front that started directly beneath the rod and grew downward. They used high-speed cameras to take 10,000 frames per second and capture a kind of shockwave that jams the cornstarch particles together and transforms the liquid into solid.

"The way it grows is similar to how a snowplow works," said Waitukaitis. "If I push a shovel in loose snow, a big pile of compacted snow grows out in front of the shovel, which makes it harder and harder for me to push."

When cornstarch is mixed with water, individual grains retain their structure and pile up when impacted with an object. They become 'jammed' and stop any movement through the liquid. The television program "MythBusters" has even shown that a full-grown man can run across a vat of this liquid mixture.

The discovery may help scientists to better understand the behaviors of other non-Newtonian fluids such as quicksand, wet concrete, Silly Putty, and the viscous material that gives your car all-wheel drive, according to Popular Mechanics. Some engineers are even investigating these suspensions as the basis for a new type of body armor, the researchers said.

"It would be liquid, so it would conform to a particular shape, and when it gets hit hard it knows it needs to become hard," Waitukaitis said. "It's a smart material, one that increases resistance with the amount of force applied against it."

But if engineers are going to be doing testing on these suspensions, they had better prepare for a chaotic lab.

"It's an incredibly messy experiment," Waitukaitis said. "I have a blue jumpsuit I wear all day. When I do these experiments, I'm totally covered in cornstarch."

Watch the video to see water turn to solid in the cornstarch tests at the University of Chicago: 

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