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.
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.