Sharks are among the fastest animals in the sea, partly due to the texture of their skin. Minute structures in their scales form a series of shallow ridges, known as riblets, which extend along their bodies from nose to tail, reducing drag and allowing them to reach high speeds without expending much energy.
It is a trick that engineers have always been keen to copy and it has been used with some success on ships and racing yachts. Polymer films with a riblet-like texture can be bonded to the hull to reduce drag and increase the speed through the water.
However, as design techniques become more sophisticated with computational fluid dynamics optimising aerodynamic forms, it is becoming more difficult to use riblets in the design.
The riblet-textured films are fine for flat or convex surfaces but the designs produced by computerised optimisation tend to have far more complex shapes, unsuitable for bonding.
Attempts to create a riblet texture on the surface have not been successful — laser-patterning and milling techniques can create the right sort of surface but as soon as a paint or lacquer is applied, it has the effect of smoothing out the surface, destroying the carefully-created texture.
Researchers at the Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research believe they have found a method for applying riblets to complex surfaces, by borrowing a technique from the printing industry. Rather than texturing the surface or applying a textured film the team, led by Volkmar Stenzel, has found a way to form riblets in the wet lacquer then fix it into place through rapid curing.
The key to the technique was the combination of materials. ‘We had to look for a tool that didn’t adhere to the lacquer, so that we could impress the required structure on to it,’ said Stenzel.
The team devised a ‘stamp’ made from silicone film, cast with the riblet pattern. The liquid resin lacquer, based on the resins used in aviation paints, is sprayed directly on to the film, which runs over three flexible rollers, then ‘printed’ on to the surface of the ship, car or aircraft.
Once transferred, the resin is cured with ultraviolet light. This process is so fast that the riblet texturing imparted by the silicone film is retained.
As well as being fast-curing, the lacquer resin is mechanically tough and ‘should also be resistant to strong UV at high altitudes’, said Stenzel.
However, he added, the technique has more potential. The silicone film is capable of holding patterns with resolutions down to nanometre-scale — similar to the light-interference patterns that make up holograms. ‘With this technology, we can apply any other micro and nano structures to lacquered surfaces,’ claimed Stenzel.
Researchers believe they have overcome the problem of replicating the ribbed surface of the shark’s skin to improve the aerodynamics of complex surfaces. Stuart Nathan reports.