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soft polymer spheres

This sphere, made of a soft polymer with a thin coating of a stiffer polymer, becomes dimpled when air is pumped out of its hollow center, causing it to shrink.

There is a reason why golf balls have that dimpled surface, as do the latest soccer balls and even some athletic wear. Testing over the years has proved that such irregular surfaces, on round or blunt objects, dramatically cut the drag caused by air resistance and thus can increase speed.

Now MIT researchers have found a way to harness that effect to reduce drag on a variety of surfaces.

Aerodynamics studies have shown that while a ball with a dimpled surface has half the drag of a smooth one at lower speeds, it has more drag than a smooth surface at higher speeds. So the ideal would be a surface whose roughness can be altered, literally, on the fly—and that’s what the team led by Pedro Reis, a professor of mechanical engineering and civil and environmental engineering, has developed.

To get surfaces that could change in real time, the researchers imitated the multilayer configuration that causes smooth plums to dry into wrinkly prunes. Reis and his team made a hollow ball of soft material enveloped by a stiff skin, both layers made of rubberlike materials. Then they extracted air from the hollow interior to make the ball shrink, causing its surface to wrinkle.

When the researchers saw the wrinkled outcomes of initial tests with their multilayer spheres, “we realized that these samples look just like golf balls,” Reis says. “We systematically tested them in a wind tunnel, and we saw a reduction in drag very similar to that of golf balls.”

Because the surface texture can be controlled by adjusting the balls’ interior pressure, the degree of drag reduction can be controlled at will. “We can generate that surface topography or erase it,” Reis says. “That reversibility is why this is pretty interesting; you can switch the drag-reducing effect on and off, and tune it.”

The team refers to its invention as “smart morphable surfaces”—or “smorphs.” The pun is intentional, Reis says: the paper’s lead author—former postdoc Denis Terwagne, a fan of Belgian comics—pointed out that one characteristic of the Smurfs cartoon characters is that no matter how old they get, they never develop wrinkles.

One possible application could be in automobiles: adjusting the texture of exterior panels to minimize drag at different speeds could increase fuel efficiency, Reis says.

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Credit: Photos courtesy of Pedro Reis

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