The Reason You Can Walk on Water (and Cornstarch)

A new explanation of the classic cornstarch experiment may help researchers to enhance bulletproof vests, design safer helmets, and build better roads.

By Sarah Fecht
Ripples in a cornstarch-water mixture after a metal rod strikes its surface.


Scott Waitukaitis

July 11, 2012 2:21 PM Text Size: A . A . A
When you mix cornstarch with water, it forms a sticky goop with weird properties. When vibrated, the goop forms writhing, fingerlike projections. You can poke a hole in it and the hole won’t close immediately. And if you punch it the starchy liquid, instead of splashing like water, turns into a solid that’s strong enough to walk on—provided you don’t stop in the middle.

This classic science-fair demo shows the power of a non-Newtonian fluid—a fluid that doesn’t obey the simple, logical laws described by Sir Isaac Newton. These materials have left scientists scratching their heads for more than a century. How does the liquid morph into a solid and then back again?

New research published in Nature this week proposes a delightfully simple explanation. "Normally, the cornstarch grains just bob around in the water," says Scott Waitukaitis, a physics Ph.D. student at the University of Chicago and study co-author. "When you hit the surface, the grains pile up like snow in front of a plow. Very quickly, you have to push against this really solid substance, and the further you push, the harder it becomes." The snowplow analogy may seem intuitive, but because simple fluids like water are difficult to compress, no one had used a compression model to explain complex fluids such as cornstarch goop.

The discovery may help scientists to better understand the behaviors of other non-Newtonian fluids such as quicksand, wet concrete, Silly Putty, and the viscous material that gives your car all-wheel drive. (Engineers take advantage of this behavior, also called shear thickening, in viscous couplings on four-wheel-drive cars.) "For these types of suspensions, I think people hadn’t really looked at it this way before," says Norman Wagner, an engineer at the University of Delaware.

In their experiment, Waitukaitis and physicist Heinrich Jaeger filled a tub with water and cornstarch, then used high-speed videography to watch as they slammed a metal rod into the mixture. The video below shows the goop solidifying around the impact site of the rod. Instead of penetrating the surface of the mixture, the rod pushes it down, forming a bowl-like depression.

And just for the fun of it, here’s a video showing a bowling ball bouncing off of the goop:

The trouble with studying this weird stuff is that it’s opaque. So the researchers used X-ray imaging to monitor what happens inside the goop during impact. As the rod smashed into the mixture, they saw that the water between the cornstarch particles seemed to flow away from the site of impact, leaving behind a very dense patch of particles. The further the rod was pushed, the denser those particles became.

"They have a really nice, simple dynamical model for it," says Michael Shelley, a New York University mathematician who studies fluid dynamics. Shelley says that having a better understanding of these types of fluids could lead to fundamentally new applications. "One reason why we can design aircraft so well is we have a really good handle on how simple fluids flow, like in aerodynamics. But we don’t have good equations for complex fluids. We don’t have detailed theories that would let you design an aircraft that could move through a complex fluid."

Non-Newtonian fluids like the cornstarch mixture are really good at absorbing and dissipating energy, which makes them potentially handy for things like filling potholes, improving automotive and aircraft safety, and building better helmets and protective gear. Wagner says the research "may have some impact, no pun intended, in the ballistic work that I do"—using fluids with similar properties to make better bulletproof vests. When coated with a fluid suspension of ceramic nanoparticles that solidifies on impact, Kevlar vests can be lightweight and more flexible but still strong.

There are still plenty of questions that need answering. Wagner says cornstarch particles are comparatively large; researchers need to find out whether the phenomenon holds for micro- and nano-size particles. Shelley says he’d like to see what the actual particles are doing while they’re in this solidified state—do they come into physical contact, or are they just held in place because there isn’t enough fluid to move around in? And once a person has run across the cornstarch mixture, why does the mixture morph back into a liquid? Why don’t the particles stay jammed up?

As one reviewer quipped: "The plot thickens."

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