Physicists foil classic oobleck science trick

This cornstarch-water mix usually turns solid upon impact, but science can now turn it back to liquid

Because oobleck solidifies when hit by footfalls, runners can cross this liquid mix of cornstarch and water. Science now shows how to sink such runners.

Brett Morrison/Beakerhead/Flickr (CC BY-NC-ND 2.0)

It takes guts to attempt running across the surface of a pool of liquid goop, even if it is oobleck. That’s a strange mixture of cornstarch and water. Yet as this and other videos show, people can jump, dance, even flip across oobleck without sinking in. But beware: sneaky physicists have just shown how to foil those runners and dancers.

All it takes to sink them is a good shaking. Researchers shared their discovery May 8 in Science Advances.

Oobleck is a non-Newtonian fluid. That means its viscosity — how thick it is — changes when force is applied to it. Like when you slap it with a hammer or your feet. Do this and liquid oobleck solidifies. Some non-Newtonian fluids behave differently. For instance, ketchup and frog saliva both get thinner when force is applied to them.

But back to oobleck. In new lab experiments, physicists dropped a cylinder onto this goop. As expected, the force of its slap onto the liquid’s surface caused particles of the cornstarch to jam into each another. That made them act as a solid. Ultimately, the cylinder sunk. But very, very slowly.

Then the researchers repeated the experiment. This time, they rapidly rotated the container holding the oobleck clockwise and counterclockwise. That made the cylinder sink pretty quickly.

Meera Ramaswamy is a physicist at Cornell University in Ithaca, N.Y. By oscillating the container, she explains, “you basically move the [cornstarch] particles so they are no longer in contact. And this makes it liquid again.”

The same effect should sink a foot impacting the surface of oobleck in a rotating tub, she and her colleagues now report. But their finding is more important than just another party trick. It might prove useful to companies that work with industrial processes that involve similar fluids. For instance, it might prevent clogs in tubes that carry cement.

The next step, Ramaswamy says, is to try the technique on a larger scale. Then her team could also test just how well it would thwart would-be runners.

A mix of cornstarch and water will thicken into a solid in response to an impact. So a cylinder hitting the surface of that mixture sinks slowly. But rotating the oobleck’s container back and forth essentially re-liquifies the mixture. Now the cylinder sinks more quickly. The same technique could be used to foil a classic physics demo: people running across the surface of oobleck.

Physics writer Emily Conover studied physics at the University of Chicago. She loves physics for its ability to reveal the secret rules about how stuff works, from tiny atoms to the vast cosmos.

More Stories from Science News for Students on Physics