Metamaterials – the term may sound esoteric to the layman. In science and engineering, however, this is an interesting field of research that has developed at a highly dynamic pace, particularly since the 1990s.
To the naked eye, a metamaterial looks like an ordinary material. On smaller scales, however, it features an unusual, carefully engineered structure, endowing it with special mechanical or physical properties that the original basic material does not possess.
Such artificially designed materials are, for example, very light, stiff, highly deformable, or they mitigate impact and attenuate vibrations. Applications range from shoe soles (as reported by ETH News) and helmets all the way to microelectronics.
Special properties thanks to a special microstructure
Dennis Kochmann, Professor of Mechanics and Materials Research at ETH Zurich has extensively worked on metamaterials in his research. “It is fascinating how, through a special microstructure, you can endow a material with special properties that it does not possess without this structure,” he explains.
Kochmann and his collaborators recently presented a novel so-called phononic metamaterial in two scientific publications – a material capable of precisely controlling mechanical waves, such as vibrations or acoustic signals.
Such a metamaterial could, for example, be deployed to harvest energy from vibrations or to process signals purely mechanically, which is of interest for sensors and mechanical computers that operate without electricity.
Wafer-thin silicon membrane as a wave guide
If a metal plate is excited to vibrate – for example, by hitting it with a hammer – these vibrations usually spread in a circular fashion, similar to surface waves in water. If this plate has a carefully designed structure, however, it can redirect waves along specific paths – and it is precisely this effect that the ETH researchers have exploited.
Instead of a metal plate, they used an extremely thin silicon membrane into which the researchers etched countless holes by way of photolithography and etching techniques, thereby forming a specific pattern.
A pattern comprising millions of elements
The pattern consists of millions of repeating square elements – minute squares, each further divided diagonally into four squares. At the centre of the main square is a four-pointed star.
Unlike in many other metamaterials, these unit cells are not identical across the entire pattern but change incrementally, as the lengths of the star’s arms vary.
The ETH researchers used custom-built computer models to generate these patterns and simulated how a wave striking the pattern propagates in rays.
“If one were to simulate the entire wave field in a conventional manner, it would be extremely computationally expensive, because the design space is huge with millions upon millions of degrees of freedom,” explains Kochmann’s former team member Charles Dorn, now an assistant professor at the University of Washington, who was in charge of the simulations.
Playing puzzle with metamaterials
“The design of our metamaterial is modular, just like a jigsaw puzzle,” as Kochmann explains. In this way, different puzzle pieces perform specific functions, such as deflecting rays at right angles or splitting waves into different directions based on their frequency. When the researchers skilfully assemble the appropriate puzzle pieces, they can generate complex wave paths, such as a figure-eight path.