Streamlined sharks are legendary for their effortless swimming. George Lauder from Harvard University, USA, explains that the fish have long inspired human engineers, but more recently attention has focused on how the fish's remarkable skin boosts swimming. Coated in razor sharp tooth-like scales, called denticles, the skin is thought to behave like the dimples on a golf ball, disturbing the flow of water over the surface to reduce the drag. But something didn't quite sit right with Lauder. 'All of the shark skin studies were done on flat shark skin mimics that were held straight and immovable. But shark skin moves', recalls Lauder. So, when Masters student Johannes Oeffner joined his lab, Lauder suggested that they take a look at the fluid dynamics of shark skin and its analogues to find out how the fish's motion affects fluid flowing over the rough surface. The duo publishes its discovery that shark skin actually generates thrust to give the fish an additional boost in The Journal of Experimental Biology at http://jeb.biologists.org/.
But first the scientists had to get hold of some fresh shark skin, so they went to a market in Boston where they found several large makos. Back in the lab, Oeffner carefully removed sections from a mako's skin and attached them to both sides of a rigid aluminium foil. Then he immersed the foil in a flow tank, reproduced the swimming motion of a fish by wiggling it from side to side and measured the rigid 'swimming' foil's speed by matching it with the flow of water moving in the opposite direction.
Having measured the foil's swimming speeds with intact skin compete with denticles Oeffner carefully sanded off the denticles and set the foil swimming again. However, instead of slowing down as the duo had expected the denticle-free foil speeded up. So the shark skin's denticle surface impeded the rigid swimmer. 'But then we remembered our premise that the sharks
|Contact: Kathryn Knight|
The Company of Biologists