How to walk on water

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Dump enough cornstarch into a swimming pool, and you can walk across the water’s surface. Scientists have now revealed the secret of this YouTube party trick.

When struck by a foot, the particles suspended in the water jam together like snow piling up in front of a snowplow. The compaction forms a hard patch that can push back with the same amount of bone-crushing pressure concentrated at the tip of a high heel, researchers report in the July 12 Nature.

“If you were to punch the suspension, you might break your wrist,” says Scott Waitukaitis, a physicist at the University of Chicago who was inspired to study the goop after watching videos of people running on top of it.

Water added to equal or larger quantities of cornstarch has long been a staple of science fair demos, thanks to the mixture’s Jekyll-and-Hyde personality. A non-Newtonian fluid, it doesn’t behave like regular liquids. A hand eased into it slides in gently, without much resistance. But smack the surface hard, and it smacks back.

To understand the forces at work, the researchers struck the mix soundly with a metal rod and monitored the repercussions. Previous experiments had rubbed the material between two plates instead — a common technique for testing the properties of liquids, but one that shears the fluid sideways instead of producing a direct impact. 

X-rays of the opaque muck revealed how the stuff moved beneath the surface. Simulations based on the data suggested that the initial impact squeezed water out of the spaces between the particles. Friction between the particles then took over. They stuck together in an expanding front that behaved like a solid, pushing back against the rod.

“I was not extremely surprised by the results,” says Daniel Bonn, a physicist at the University of Amsterdam who suspected a similar mechanism after shooting bullets into cornstarch suspensions. “But the experiment was interesting because they’re able to look inside the cornstarch as it thickens.”

Both Bonn and Waitukaitis hope that understanding how cornstarch behaves will help other researchers who are trying to make “liquid” body armor by soaking Kevlar in similar suspensions. But they both caution that what happens in one suspension may not happen in another.

Despite decades of study, no one yet understands why cornstarch thickens when perturbed but quicksand and ketchup thin, even though all are simply particles suspended in liquids.

An aluminum rod striking a mix of cornstarch and water fails to penetrate as particles in the ooze jam together like snow compacted by a plow.
Credit: Scott Waitukaitis/Univ. of Chicago