George Lauder in Harvard Magazine:
Sharks present an interesting case study because unlike other fast swimmers such as tuna or swordfish, which have smooth skin surfaces, shark skin is rough—covered in teeth-like structures called denticles. Although ichthyologists have known for decades that denticles likely hold the key to a shark’s ability to quickly and efficiently move through the water, how they contribute to speed remained a mystery.
Lauder’s laboratory ran a series of experiments last summer that used small pieces of shark skin to explore how they interact with water. Samples were placed in tanks that move water over the skin at a known rate. The researchers added particles to the water so they could see with microscopic imaging systems how it flowed over the denticles. When they analyzed the resulting data to calculate the friction at the interface between skin and water, they found that the water flow created small fluid vortices that reduced drag. Lauder compares the phenomenon to the dimples in a golf ball, which enable golfers to hit the ball about twice as far as a completely smooth ball. “You would think that you would want to be as smooth as possible to be most effective at moving through a dense fluid like water,” he says, “but actually you don’t want to be as smooth as possible. That roughness really matters for efficiency.” Lauder and his colleagues are now using their findings to “print” artificial denticles with properties similar to shark skin that could one day coat the surface of underwater robots to help make them more efficient swimmers.
More here.
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