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Surfaces and interfaces

Surfaces and interfaces

Reptile skin inspires super-slippery steel surfaces

07 Jul 2015 Hamish Johnston
Slippery scales: the two low-friction surfaces

The skins of two slithery reptiles – the ball python and a type of lizard known as the sandfish skink – have inspired researchers to create a new kind of super-slippery biomimetic material. By etching patterns similar to those found on these creatures into the surface of steel, Christian Greiner and Michael Schäfer of the Karlsruhe Institute of Technology were able to cut friction by as much as 40%. The researchers say that the work could help to minimize friction in tiny mechanical devices where lubricants cannot be used.

The skin of some snakes and lizards is unusual in that it is slippery when the creature moves forward but resistant to movement in the opposite direction. Apart from allowing the creatures to propel themselves forward, this low friction associated with forward motion combined with the skin’s high resistance to wear has made it an attractive model for researchers seeking to develop new materials. In 2012, for example, scientists were inspired by the skin of the sandfish to create a material that is highly resistant to wear by sand and other particles.

Overlapping scales

Greiner and Schäfer created their reptile-inspired patterns on flat steel surfaces 7.5 mm in diameter using a technique called laser surface texturing. The two patterns they studied were inspired by the overlapping scales found on the python and sandfish, with each scale being oval shaped and about 50 μm long.

The scales protrude about 5 μm from the surface and overlap each other to form columns. In one pattern the columns are isolated from each other, whereas in the other pattern the columns overlap each other (see figure above).

The patterned surfaces were then slid across a smooth, dry sapphire surface at a constant speed of 0.1 m/s and downward force of 2 N. When compared with a smooth steel surface, the isolated columns had 40% lower friction, whereas the overlapping columns had a 22% reduction. The researchers had expected friction to be lower because the species they mimicked live in dry environments and do not secrete oils or other liquids onto their skin. However, they were amazed by the size of the reduction.

Leaping forward

“If we’d managed just a 1% reduction in friction, our engineering colleagues would have been delighted; 40% really is a leap forward and everyone is very excited!” says Greiner. Indeed, when the textured surfaces were lubricated with mineral oil, they experienced greater friction than did a smooth lubricated surface.

The researchers believe that their discovery could help to reduce friction in machines that cannot be lubricated. These include nanometre and micron-sized devices in which lubricants tend to gum up moving parts, rather than help them move. Potential applications include reducing friction in the sensors used in anti-lock braking systems, computer hard-disk drives, accelerometers used in mobile phones and machines that operate under vacuum conditions.

Unlike reptile skin, which has low friction when moving in only one direction, the new textured surfaces have reduced friction in at least two directions. Surfaces with unidirectional friction reduction could be used to create snake-inspired robots that would be useful for exploring extremely dusty environments on Earth or even in space. Greiner is currently trying to develop textured polymer surfaces that mimic the unidirectional nature of reptile skin.

The research is described in Bioinspiration & Biomimetics.

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