Cornell researchers simulated the impact of a human foot on a thickening cornstarch suspension by dropping a cylinder onto its surface. They repeated the experiment with a bottom plate applying a form of a rotational oscillation, called orthogonal shear, which de-thickened the suspension and made the cylinder sink faster. 

A simple twist could sink cornstarch walkers

The videos began appearing online a decade ago, racking up millions of views: people mixing cornstarch and water in a shallow pool and walking or running across the surface.

The improbable physics feat could be chalked up to a fascinating trick of fluid mechanics, whereby dense suspensions solidify when impacted. Now, a Cornell team is one step closer to engineering a new hurdle for these cornstarch crossers that would make them sink in their tracks.

The team’s paper, “Tunable Solidification of Cornstarch under Impact: How to Make Someone Walking on Cornstarch Sink,” published May 8 in Science Advances. Postdoctoral researcher Ran Niu and doctoral student Meera Ramaswamy were lead authors.

The team, led by Itai Cohen, professor of physics in the College of Arts and Sciences, previously showed how to manipulate – or tune – the shear viscosity of thickening fluids, a class of materials that can increase their resistance to flow more than a hundredfold when stressed.

The new project comes with a literal twist.

The researchers used a form of a rotational oscillation, called orthogonal shear, to tune the solidification of thickening fluids under compression and extension (squeezing and stretching, basically).

While traditional shear essentially reorganizes the particles suspended in the fluid, compression and extension affect the suspension volume, which gave the researchers an opportunity to explore how these materials solidify.

“We still think these forces arise from networks of contacting particles and that by jostling those networks, we can melt them and liquefy that suspension,” Cohen said. “What this requires is a slightly different geometry to the applied melting flows. And that’s what Meera was able to come up with, this rotational oscillation around the impacting or extending plate.”

The team simulated the physical action of a human foot stepping on a cornstarch-water mixture by dropping an impacting plate on a surface of a thickening cornstarch suspension while a bottom plate applied the oscillating force.

The experiment demonstrated that researchers could completely de-thicken the suspension, turning it from a solid to a liquid, and alter the sinking speed of the plate dramatically.

“If we just drop the cylinder onto the surface, it basically solidifies the suspension under it, and takes a very long time to sink,” Cohen said. “But if we apply these orthogonal shear oscillations around the cylinder, then we’re able to completely de-solidify that flow and the cylinder drops through the material like it’s a liquid.”

While there may not be many practical applications for orthogonal shearing in this context, Cohen hopes his team can develop a method to control the process with acoustic energy. The use of acoustics could benefit applications that require fluids to be tuned as they move through a stationary pipe, such as in 3D printing, food processing and manufacturing concrete.

“There’s lots of flow geometries where you’re trying to suck fluid out or flow fluid in, where you have compressive and extensive flows, and you’ll need to figure out how to de-solidify these suspensions. This gives you a way to do that,” Cohen said. “If we can show that acoustics can do the same thing as our oscillating flows, then we’re in business. This work, at least, is a proof of principal that some kind of mechanical perturbation can do the trick.”

This discovery also means that someday you might see a video of someone walking across a layer of cornstarch and water, and with the flip of a switch, their viral stunt may no longer be on solid ground.

Researchers from the University of Edinburgh and Anton Paar contributed to the paper. The research was supported by the National Science Foundation and Anton Paar’s VIP academic research program.

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Gillian Smith