Mako sharks can swim as fast as 70 to 80mph, earning them the moniker "cheetahs of the ocean." Now scientists at the University of Alabama have determined one major factor in how mako sharks are able to move so fast: the unique structure of their skin, especially around the flank and fin regions of their bodies. The team described their work at the American Physical Society's 2019 March meeting this week in Boston.

University of Alabama engineer Amy Lang conducted a series water tunnel experiments in her lab to test samples of real mako shark skin from the animal's flanks, using a technique called particle image velocimetry to measure the velocity of the water flowing over and around the skin. Anyone who has touched a shark knows the skin feels smooth if you stroke from nose to tail. Reverse the direction, however, and it feels like sandpaper. That's because of tiny translucent scales, roughly 0.2 millimeters in size, called "denticles" (because they strongly resemble teeth) all over the shark's body, especially concentrated in the animal's flanks and fins. It's like a suit of armor for sharks.

Mako sharks have evolved a distinct passive "bristling" aspect on some of their scales to swim faster. Lang's lab coordinated their project with biologists at the University of South Florida, who imaged shark skin and mapped out the scales, noting particularly how many of the scales were capable of this passive bristling and the angles at which such bristling occurs. They found that near regions like the nose, the scales aren't especially flexible, more like molars embedded in the skin. But near the flanks and fins, the scales are much more flexible.

The scales "are literally like little loose teeth sitting in the skin."

"They're literally like little loose teeth sitting in the skin," said Lang. "But they're not just sitting there, they have a certain orientation. When the [water] flow goes from left to right [as the shark swims], it does not bristle the scales, but when the flow reverses, you get bristling."

That has a profound effect on the degree of pressure drag the mako shark encounters as it swims. Pressure drag is the result of flow separation around an object, like an aircraft or the body of a mako shark as it moves through water. It's what happens when the fluid flow separates from the surface of an object, forming eddies and vortices that impede the object's movement.

"It's like when you stick your hand out the window [of a moving car] and feel the high pressure on the front of your hand versus the low pressure on the back of your hand," said Lang. "And the first line of defense against pressure drag is to streamline the body." Since the shark's body is constantly undulating as it swims, "it needs something to help keep the flow attached around that body to reduce that drag."

That something is its denticles, which can flex at angles more than 40 degrees from its body—but only in the direction of reversing flow (i.e., from tail to nose). This controls the degree of flow separation, similar to the dimples on a golf ball. The dimpling, or scales in the case of the mako shark, help maintain attached flow around the body, reducing the size of the wake. According to Lang, a dimpled golf ball will travel 30 percent farther when hit than if the same ball were smooth. The same pressure drag reduction holds true for smooth and scaled shark skin.

One thing that doesn't seem to play much of a role in the effect is the undulations of the shark's body as it swims, since Lang et al. observed a reduction in pressure drag even when the skin was mounted flat. The unique skin structure also doesn't work as well in air; it needs to be wet. "Our hope as engineers is, if we can isolate this mechanism, we can 3D print the same kind of surface to get a similar effect in air," said Lang.

The research could one day lead to new designs capable of reducing drag on aircraft or helicopters, among other potential applications—possibly even high-tech swimsuits for professional athletes. The current swimsuit technology involves adding tiny grooves to create what Lang terms a "riblet" effect, which reduces a different kind of drag—turbulent skin-friction drag, the result of air rubbing up against a surface. Lang's research could improve such suits further by reducing pressure drag, but "as far as I know, I've never heard of a swimsuit manufacturer trying to replicate that."