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Harvard Study Reveals New Shark Skin Properties

Research under more natural conditions details unexpected findings.

By David Alderton | February 14, 2012

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denticles on shark skin

Shark skin with denticles showing at left, above the eye. Photo credit: David Alderton

There is growing interest in the way in which animals have developed particular characteristics to help them move more efficiently, with this field of study being described as biomechanics. Sharks as an example are aided in terms of their movement by their remarkable skin. With its sharp tooth-like scales known as denticles, this was believed to behave like the dimples on a golf ball, disturbing the flow of water over the surface to reduce the drag.

Flawed studies
But George Lauder from Harvard University  was not convinced by this explanation, bearing in mind the way in which research had been conducted. ‘All of the shark skin studies were done on flat shark skin mimics that were held straight and immovable. But shark skin moves” explains Lauder. Working with masters student Johannes Oeffner, Lauder decided to investigate the fluid dynamics of shark skin and its analogues further, in a more natural situation. The aim of the duo was to discover how the way in which the fish moves actually impacts on the water as it comes into contact with this rough surface.

Oeffner therefore took skin sections from a mako shark purchased in a Boston market, and attached this to both sides of rigid aluminium foil. He immersed the foil in a flow tank, and replicated the shark’s swimming motion by wiggling it from side to side. He was then able to measure the rigid foil’s speed as it moved, by matching it with the flow of water moving in the opposite direction.

Having recorded the foil’s swimming speeds with intact skin – complete with denticles – Oeffner carefully sanded off the denticles and set the foil swimming again. Now though, rather than slowing down – as the researchers 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 aren’t rigid” said Lauder. “So how would the shark skin perform when flexing like a real fish?”

Variable performances
Gluing two pieces of shark skin together to produce a flexible foil, Oeffner then repeated the swimming experiment, and this time, the denticles had a dramatic effect. The intact skin foil swam 12.3% faster than the sanded skin. The shark’s rough surface improved the swimming performance spectacularly.

However, when the scientists tested the swimming performance of two shark skin mimics – a sharp-edged riblet design and the famous Speedo® Fastskin® FS II fabric – they were in for a shock. Although the riblet surface improved the flexible foil’s swimming speed by 7.2%, the dented surface of the Speedo® fabric had no effect at all. Lauder points out that figure-hugging Fastskin® swimming costumes probably enhance the swimmer’s performance in other ways.

After proving that the denticles on shark skin significantly improve the fish’s propulsion, Lauder and Oeffner were keen to find out how they affect fluid flows around the body. Returning the flexible shark skin foil to the swim tunnel, and capturing the water’s swirling motion with laser light, it became clear that in addition to reducing drag, the skin was actively generating thrust.
“That’s the number one surprise. It’s not just the drag-reducing properties, but the denticles alter the structure of flow near the shark skin in a way that enhances thrust” explains Lauder. His aim is now to design physical models that will reveal how altered denticle arrangements affect fluid flows over the skin. He also plans to build a computational model, with the aim of distinguishing between the beneficial effects of the skin’s thrust and its drag reduction.

Reference: Johannes Oeffner, George V. Lauder. The hydrodynamic function of shark skin and two biomimetic applications. The Journal of Experimental Biology, 2012; DOI: <10.1242/jeb.063040>
 

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Harvard Study Reveals New Shark Skin Properties

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Reader Comments

Carl    Livermore, CA

2/14/2012 11:03:26 AM

Something that could have a market in the human world.

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