Why you can walk on water (mixed with cornstarch)

There’s a well-known experiment in which cornstarch is added to water, turning it into a rather odd substance known as oobleck.

It’s liquid – but if you hit it hard it turns into a solid. It’s even possible to run across it, sinking only when you stop.

The trouble is that nobody’s ever been able to explain the phenomenon fully. Whie the push-pull of shearing forces would cause the suspension to thicken, they’re a long way short of being able to account for the large forces needed to keep an adult high and dry.

But Scott Waitukaitis and Heinrich Jaeger of the University of Chicago say they’ve shown that compressive forces can generate a rapidly growing, solid-like mass in the suspension.

“We found that when you hit the suspension, a solid-like column grows below the impact site,” says Waitukaitis.

“The way it grows is similar to how a snowplow works. 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.”

In the same way, individual cornstarch grains pile up in front of the impacting object and become temporarily jammed.

“It’s an incredibly messy experiment,” says Waitukaitis. “I have a blue jumpsuit I wear all day. When I do these experiments, I’m totally covered in cornstarch.”

Amazingly, Waitukaitis believes that oobleck could be used as the basis for a new type of body armor.

“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,” he says.

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