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Friday, July 25, 2014

Tomorrow’s Fastest Cars Could Be Covered in Morphable Skins

As reported by WiredWrinkles aren't usually an aspect of the future that gets people excited. But fast cars are. And someday we might have cars that can accelerate more quickly, and efficiently, by morphing their surface texture through the mechanics of wrinkling.
Speed-enhancing body wrinkles on your Tesla are still years away, but researchers at MIT have created what could be the first step: a ball with morphable surface texture. They were able to get their creation, which they call a smorph (short for smart morphable surface), to wrinkle into a dimpled pattern similar to a golf ball’s, with similar aerodynamic properties.
Smorphs are sort of like raisins. As the soft inside of a grape dries out, the stiffer skin can’t shrink with it. Instead, it develops wrinkles to conform around the reduced volume. Smorphs don’t dry out (they also make terrible snacks), but the volume of  a smorph can be similarly reduced by sucking air out of its hollow core. That core is surrounded by different polymers: a thick, squishy layer covered by relatively stiff outer skin. As the core shrinks, the squishy layer is soft enough to contract smoothly, but the skin is forced to wrinkle.
The trick is controlling exactly how a smorph wrinkles. MIT mechanical engineer Pedro Reis, the material’s lead inventor, studies how wrinkling and other types of structural failures can be made useful. He says the first step toward controlling the wrinkling of a smorph is making the squishy base layer thick enough that the sphere doesn’t crumple like a ping pong ball. From there, they can tune the pattern of the wrinkles by changing the thickness of the outer skin. Dimples form when the skin is one-tenth to one-hundredth of the sphere’s radius.
The dimpling isn't a pre-designed pattern. It's the natural way the Smorphs' skin collapses when air is sucked out of the middle.
The dimpling isn’t a pre-designed pattern. It’s the natural way the Smorphs’ skin collapses when air in the center is sucked out.  Pedro Reis/MIT
Because the smorph dimples look so much like those on a golf ball’s surface, the researchers were inspired to test their creation in a wind tunnel. It’s well established that a golf ball’s dimples help it fly further. Air passing over the dimples creates a bunch of tiny vortices. Rather than slowing the ball down, these vortices create a thin, turbulent sheath that the surrounding air can’t cling to. The result is lower drag. 
Sure enough, when the researchers tested the smorph in a wind tunnel, they found that it was about twice as aerodynamically efficient when dimpled.
But the sheath of vortices only forms at relatively low speeds. If a golf ball were to fly fast enough, it would be better off with a smooth skin. This is where smorphs could offer a huge advantage.
“What our system lets you do is tune the drag between the two extremes,” Reis said. Because of their size, golf balls rarely reach a speed when the dimples are less efficient. But something bigger, like a car, could be more fuel efficient with a few strategically placed morphable surfaces that would be dimpled at slower speeds and smooth when the car speeds up.
Earlier this year, Reis won an NSF grant to keep developing smorphs, which he hopes to someday scale up to use on cars, aircraft, and even buildings. He’s optimistic, but says this is probably a long way off. One problem is that hexagonal dimples are unstable on flat surfaces. So far smorphs have only been used on a round, ball shape, but Reis and his co-authors believe they can figure out how to reproduce the pattern on slightly curved surfaces. Creating the same aerodynamic dimpling on a car’s complex curves will be even more challenging.