http://youtu.be/qFMwqGucfvw What's something that everyone hates? That's the question that undergrads at Case Western University asked recently while brainstorming their entry for a materials science competition. Their answer: potholes. And their answer to the problem of how to fill them cheaply and easily? Basically, corn starch and water. It's not as strange as it sounds: the corn starch putty is a non-Newtonian fluid, a class of fluids that behave very differently from water. In the case of the putty, when it's placed in an oddly shaped receptacle, like a pothole, it will flow like a liquid into all the nooks and crannies. But the second you push on it, with a car, for instance (or, as you can see in the above video, your feet), it turns solid, resisting compression and giving drivers a smooth ride. Here's a little more on the physics involved, courtesy of ScienceNOW, including how ketchup-like non-Newtonian fluids are different from putty-like non-Newtonian fluids:
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.
Now, the physics is snazzy, but could bags full of putty beat regular old asphalt when it comes to patching potholes? The team thinks they could be a useful stop-gap, since no special equipment or training is required to lay a bag of the stuff in a hole. Any city employee with a few in their car could stop and at least temporarily fill a hole, instead of waiting for a team of workers to arrive. Read more at ScienceNOW.
