Researchers at FOM-institute AMOLF and the Leiden Institute of Physics (LION) have developed a rubber rod with strange bending behaviours. Beyond a certain point, it bends more under decreasing pressure. This behaviour doesn’t fit our expectations and does not conform to secular laws that predict the bending process. The rod is made out of metamaterial – material with special properties that are not found in nature. By providing the rod with a carefully selected pattern of small holes, the researchers managed to induce the strange behaviour. They publish their work, a collaboration with Harvard University, in Physical Review Letters on July 20. 

The metamaterial makes up a rubber rod of about twenty centimeter containing a pattern of elliptically shaped holes. These holes give the metarod its special property. The physicists noticed that a self-amplifying effect occurs at a turning point under a certain pressure level: the rod keeps bending further, while the pressure decreases. Group leader Martin van Hecke: ‘Imagine pushing a car forward. You expect to make the car go faster by pushing harder, but in this case the car speeds up if you push less.’

Custom-made bending

The researchers started looking for a mechanism that could explain this counter-intuitive effect. They discovered that you can easily squeeze together the metamaterial with a slight push, but it strongly resists stretching when you pull. Regular materials without holes show no significant difference between pushing and pulling, unless extremely high pressures are at play. This sensitivity to the difference between pulling and pushing causes the strange effect during bending of the metamaterial rod. The shape and place of the holes precisely determine the moment this effect occurs. So by changing the hole pattern, the scientists can custom-make materials with specific properties.

Ancient building principles

The development of the metamaterial rod challenges century-old building principles. The relationship between bending and pressure was established 250 years ago by Leonhard Euler, in his universal law of elastic instability of rods. Since then, his theory has formed the basis for building houses and bridges. Euler assumed that materials only show a distinction between pushing and pulling under extreme pressure. The new metamaterial rod displays a different behaviour, opening doors for new developments. ‘Building bridges with our metamaterial rod is never going to happen,’ says van Hecke. ‘But I can imagine a robot arm that is able to bent in a clever way thanks to mechanical switches that are based on this new material.’