

Video: Watch a projectile shatter a thin layer of cornstarch Video: Watch a projectile shatter a thin layer of cornstarch

Oobleck turns solid underfoot (Image: Rory MacLeod)

WALKING on water is possible – just as long as it contains corn starch. Now it seems this miracle mixture, dubbed oobleck, can also shatter like glass. Knowing how and why could help guide its use in soft body armour and car suspensions.

Oobleck gets its name from the artificial gloop that falls from the sky in the Dr Seuss book Bartholomew and the Oobleck. It has a split personality because the corn starch exists as a solid suspended in liquid water. Gently poke real-life oobleck and your fingers easily slip through, but slap it and it suddenly stiffens.

Along with ketchup and toothpaste, which jam up if you squeeze their containers hard, it is an example of a non-Newtonian fluid. “They can be solid if you make them flow too fast,” says Matthieu Roché of the University of Paris-South. Oobleck is the most extreme variety, capable of supporting a person running across a pool, though they will sink if they slow down.


Last year, Heinrich Jaeger and colleagues at the University of Chicago discovered why oobleck behaves this way – the water flees the point of impact faster than the starch, leaving the solid particles behind. These jam up, forming a matrix of surprising strength.

While based at Princeton University, Roché and colleagues probed another property of oobleck – its tendency to break up. They spread a layer over a sheet of perspex (plexiglass) and filmed from underneath as they dropped a 300-gram, tungsten carbide rod onto the oobleck from varying heights. Because oobleck is so flexible, Roché expected it to tear like a soft metal, a process known as ductile fracture. Instead, about 6.7 milliseconds after impact, the oobleck formed pointy-tipped cracks like fracturing glass or plaster, his team reports in Physical Review Letters (doi.org/k7m). Unlike glass, though, the cracks sealed up quickly.

The team was also able to measure the speed of the cracking and discovered that cracks only appeared when the oobleck layer was below a certain thickness. Thicker layers may not crack because oobleck underneath stays liquid, absorbing the impact more easily, says Roché. Surprisingly, the force of the hit had less impact on whether the oobleck cracked than layer thickness.

Non-Newtonian fluids are already found in liquid armour, which is flexible like fabric but stiffens suddenly to fend off an incoming bullet or knife. The fluids are also being investigated for use in cars, where they could dampen shocks without requiring additional energy. The new details of how cracking occurs should improve strategies for avoiding fracture in these scenarios and other planned ones, says Roché.

Jaeger, who was not involved in the latest work, agrees with Roché: “This is a beautiful piece of research. Understanding this behaviour better will enable us to control it.”

This article appeared in print under the headline “Miracle gloop shatters like glass”