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Prototype. Waterproof bags filled with a shear-thickening fluid that resists compression when run over by traffic.

Gretchen Cuda Kroen

Silly Putty for Potholes

CLEVELAND, OHIO—So-called non-Newtonian fluids are the stars of high school science demonstrations. In one example, an ooey-gooey batter made from corn starch and water oozes like a liquid when moved slowly. But punch it, or run across a giant puddle of it, and it becomes stiff like a solid. Pour it on top of a speaker cone, and the vibrations cause the fluid to stiffen and form strange tendril-like shapes. Now, a group of college students has figured out a new use for the strange stuff: filler for potholes.

The students, undergraduates at Case Western Reserve University in Cleveland, devised the idea as part of an engineering contest sponsored by the French materials company Saint-Gobain—and took first prize last week. The objective was to use simple materials to create a novel product.

"So we were putzing around with different ideas and things we wanted to work with—and we were like, what's a common, everyday problem all around the world that everybody hates?" explains 21-year-old team member Curtis Obert. "And we landed on potholes." He and four other students decided on a non-Newtonian fluid as a solution because of its unusual physical properties. "When there's no force being applied to it, it flows like a liquid does and fills in the holes," says Obert, "but when it gets run over, it acts like a solid."

Case student Nich Barron demonstrates the pothole fillers.
Credit: Mayank Saksena

Scientists call liquids that behave like this non-Newtonian because the viscosity, or resistance, of the fluid changes in response to the forces that are applied to it. This is in contrast to Newtonian fluids, which maintain their fluid state regardless of disturbances.

There are plenty of familiar non-Newtonian fluids, says Michael Graham, a chemical engineer not involved in the project who studies non-Newtonian fluid behavior at the University of Wisconsin, Madison. Mayonnaise, ketchup, silly putty, and even blood are examples. Although these substances seem very different, Graham points out that they all contain some sort of particle—and the interaction of those particles explains their behavior.

Ketchup and mayonnaise are shear-thinning fluids. When sitting on your counter, they are thick and clumpy and don't flow because the particles have a tendency to stick together at rest, explains Graham. "Ketchup is actually mashed up tomatoes, and it's the little particles of tomato that are interacting with one another and keeping the fluid from moving," he says. "Mayonnaise is droplets of fat that stick together." But pressing on a glob of mayonnaise with a knife or shaking a bottle of ketchup creates shear stresses that disrupt the particles, so the fluids become runnier and more spreadable.

The type of material the students chose is the opposite of ketchup and mayonnaise. It's shear-thickening, meaning that when a shear stress is applied—say by the force of a car tire—it becomes stiffer and resists flowing. That's because the particles slip and slide past each other easily when moved gently, but they get stuck when strong forces are applied. "The harder you push on it, the higher the viscosity gets. If you push it really rapidly, the particles in the corn starch don't have time to rearrange and get around one another and they jam up," says Graham.

When fluid is moving quickly it is said to have a high "shear rate." At relatively low shear rates (i.e. when the fluid is disturbed gently), repulsive forces between the particles prevent them from clumping together and keep them evenly distributed throughout the fluid. However, when the shear forces that push the particles together become larger than the repulsive forces keeping them apart, the particles temporarily cluster together and form small chains called hydroclusters. Unlike individual particles, which can easily move around each other, the hydroclusters get locked in place and can't move, making the fluid temporarily behave like a solid.

The students say a little experimentation was required to get just the right formulation. "By working with different size particles, you can get different viscosities from it," says Obert. What they came up with is a powdered mixture that is stored in specially designed waterproof bags, which are made of a strong fiber like Kevlar lined with silicone. To produce a ready-made pothole patch, city workers would simply add water and seal the bag.

Currently, potholes are repaired by packing them with asphalt, which is messy, smelly, time-consuming, and requires specialized personnel and equipment, says Obert. By contrast, the fluid filled bags can be carried around in the trunks of police cruisers or vans and dropped into potholes on the spot by employees with little training or experience. They would then be covered with black adhesive fabric so that drivers don't perceive them as a hazard. "We definitely don't want people avoiding them," says team member Mayank Saksena.

The students have road-tested their designs on a number of Cleveland's potholes and found that the bags continue to perform well after more than a week of continuous use in high-traffic areas. Although the product has yet to be field tested in an actual Midwest winter, the students say the bags are intended to be sturdy enough that they can stand up to salt and freezing conditions for weeks at a time, until damaged roads can be permanently fixed. Furthermore, when the roads are repaired, the bags can be removed and reused. When they are not needed, they can be stored empty and refilled by mixing additional powder with water, for a very low cost.

The upfront price of the bags may be as much as or more than traditional repair methods, says Obert, but in the long run cities will save on materials and labor because the filling material is very inexpensive. "The bag might cost a hundred dollars but you can reuse it a hundred times, and by that time you'd be saving a ton of money."

The students plan to patent their invention, so they won't divulge their exact formulation, but they say it's biodegradable and safe enough to eat— although not very tasty. If the bags leak or tear, the contents pose no danger to people or the environment.

The city of East Cleveland has offered to help the students test their new pothole fillers, and the students say they have already been approached by several companies interested in working with them.

East Cleveland Service Director and City Engineer Ross Brankatelli says the product could be a great quick fix for temporary safety hazards, but he's less sure of its longevity in winter road conditions. "I think it will hold up under traffic—I think that part will work. But whether it will be able to handle real winter temperatures and be cost competitive as a semipermanent fix, I have some reservations about that."

Still, one thing is certain: If and when a commercial product is ready, there are more than enough holes to fill.