Physicists have found a way to foil a classic oobleck science trick



It takes guts to attempt running across
the surface of a liquid. Even more so if a sneaky physicist is nearby.

A mixture of cornstarch and water known
as oobleck solidifies when hit with a forceful impact. That effect makes for a classic science party trick, in which participants run across the liquid’s
surface (SN: 7/16/12). But a new
technique could sink those runners, researchers report May 8 in
Science Advances.

Oobleck is a non-Newtonian fluid,
meaning that its viscosity changes depending upon the forces exerted on it. Other
non-Newtonian fluids include ketchup and frog saliva
(SN: 1/31/17), both of which get
thinner with applied force, in contrast to oobleck.

In laboratory experiments, a cylinder
dropped onto the surface of oobleck sank more quickly when researchers rapidly
rotated the mixture’s container clockwise and counterclockwise. Normally, the
impact of the cylinder would cause particles of cornstarch to come into contact
with one another, jamming up into a solid. But by oscillating the container, “you
basically move the particles so they are no longer in contact, and this makes it
liquid again,” says physicist Meera Ramaswamy of Cornell
University.

In response to an impact, a mixture of cornstarch and water thickens into a solid. So a cylinder hitting the surface of that mixture sinks slowly. But when the container is rotated back and forth, it essentially reliquifies the mixture and the cylinder sinks more quickly. The same technique could be used to foil a classic physics demo: people running across the surface of the liquid.

The same effect, Ramaswamy and colleagues
say, would sink a foot impacting the surface of oobleck in a rotating tub. It
could also be useful in industrial processes involving similar fluids, for
example, preventing clogs in tubes that carry cement.

The next step, she says, is to try the technique on a larger scale, in hopes of foiling would-be runners.

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