Science & technology | Biomimetics

Not a scratch

Scorpions may have lessons to teach aircraft designers

Sand? I spit on it

THE north African desert scorpion, Androctonus australis, is a hardy creature. Most animals that live in deserts dig burrows to protect themselves from the sand-laden wind. Not Androctonus. It usually toughs things out at the surface. Yet when the sand whips by at speeds that would strip paint away from steel, the scorpion is able to scurry off without apparent damage. Han Zhiwu of Jilin University, in China, and his colleagues wondered why.

Their curiosity is not just academic. Aircraft engines and helicopter rotor-blades are constantly abraded by atmospheric dust, and a way of slowing down this abrasion would be welcome. Dr Han suspects that scorpions may provide an answer. As he writes in Langmuir, he has discovered that the surface of Androctonus's exoskeleton is odd. And when that oddness is translated into other materials it seems to protect them, as well.

Dr Han's investigations began by scouring the pet shops of Changchun, where the university is located, for scorpions. Having obtained his specimens, he photographed them under a microscope, using ultraviolet light. This made the animals' exoskeletons, which are composed of a sugar-based polymer called chitin, fluoresce—thus revealing details of their surface features. The team found that Androctonus armour is covered with dome-shaped granules that are 10 microns high and between 25 and 80 microns across. These, they suspected, were the key to its insouciance in the face of sandstorms.

To check, they took further photographs. In particular, they used a laser scanning system to make a three-dimensional map of the armour and then plugged the result into a computer program that blasted the virtual armour with virtual sand grains at various angles of attack. This process revealed that the granules were disturbing the air flow near the skeleton's surface in ways that appeared to be reducing the erosion rate. Their model suggested that if scorpion exoskeletons were smooth, they would experience almost twice the erosion rate that they actually do.

Having tried things out in a computer, the team then tried them for real. They placed samples of steel in a wind tunnel and fired grains of sand at them using compressed air. One piece of steel was smooth, but the others had grooves of different heights, widths and separations, inspired by scorpion exoskeleton, etched onto their surfaces. Each sample was exposed to the lab-generated sandstorm for five minutes and then weighed to find out how badly it had been eroded.

The upshot was that the pattern most resembling scorpion armour—with grooves that were 2mm apart, 5mm wide and 4mm high—proved best able to withstand the assault. Though not as good as the computer model suggested real scorpion geometry is, such grooving nevertheless cut erosion by a fifth, compared with a smooth steel surface. The lesson for aircraft makers, Dr Han suggests, is that a little surface irregularity might help to prolong the active lives of planes and helicopters, as well as those of scorpions.

This article appeared in the Science & technology section of the print edition under the headline "Not a scratch"

A fistful of dollars

From the February 4th 2012 edition

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