Dimples Are Cute, But Can They Also Save Fuel?

Golf balls aren't dimpled for mere cosmetic reasons. The pattern of tiny divots actually reduces drag by cutting air resistance in half.

But no matter how powerful your swing, there's only so hard you can hit a golf ball, and just so fast you can make it fly. In fact, a golf ball actually can go too fast to take advantage of its dimples because at high speeds, the dimples' effect reverses.

So researchers at the Massachusetts Institute of Technology have been looking at ways to give dimples to objects that are exposed to air resistance. This would be a valuable fuel-saver for vehicles, which create their own wind, or stationary structures such as flexible domes that can be brought down by high winds.

Their challenge was to have the dimples present at lower speeds and to smooth them out at will at higher speeds. And it appears they've succeeded, according a paper on their work published in the journal Advanced Materials.

The MIT team used a multilayer material that's stiff outside and soft inside. The researchers used a malleable material similar to rubber to create a soft ball with a stiff skin, and then sucked the air from the interior, causing the ball to shrink and its outer layer to wrinkle. And because the surface is curved, the wrinkles are dimple-shaped.

But why wouldn't a smooth surface lead to less drag and a dimpled surface lead to more drag? The MIT researchers discovered that the irregular surface holds on to a very thin "boundary" layer of air that, at lower speeds, reduces turbulence, or wake, behind the ball. The greater the wake, the more drag.
By adjusting the interior pressure of the ball, the surface texture, and therefore the drag, can be controlled at will. The MIT team calls these changeable surfaces "smart morphable surfaces," or just "smorphs."

Reis says this pun is intended. The lead author of the research paper, Denis Terwagne, is a fan of Belgian comics, including Smurfs, the Belgian comic and cartoon characters.

Reis notes that, according to Smurf lore, the blue characters never develop wrinkles, no matter how old they get.