New metamaterial can switch between hard and soft at the surface

When a material is made, you typically cannot change whether that material is hard or soft. But a group of researchers at the University of Michigan (U-M) have now developed a new way to design a ‘metamaterial’ that allows the material to switch between being hard and soft without damaging or altering the material itself.

Metamaterials are man-made materials that get their properties – in this case, whether the material is hard or soft – from the way the material is constructed rather than the material that constructs it. This allows researchers to manipulate a metamaterial's structure in order to make the material exhibit a certain property.

In the group's study, published in a paper in Nature Communications, the U-M researchers found a way to compose a metamaterial so that it can be easily manipulated to increase the stiffness of its surface by orders of magnitude – the difference between rubber and steel. Since these properties are ‘topologically protected’, meaning that the material's properties come from its total structure, they're easily maintained within the bulk material even as its surface shifts repeatedly between hard and soft states.

"The novel aspect of this metamaterial is that its surface can change between hard and soft," said Xiaoming Mao, assistant professor of physics. "Usually, it's hard to change the stiffness of a traditional material. It's either hard or soft after the material is made."

"The novel aspect of this metamaterial is that its surface can change between hard and soft. Usually, it's hard to change the stiffness of a traditional material. It's either hard or soft after the material is made."Xiaoming Mao, University of Michigan

For example, a dental filling cannot be changed after the dentist has set the filling without causing stress, either by drilling or grinding, to the original filling. A guitar string cannot be tightened without putting stress on the string itself, according to Mao.

Mao says the way an object comes into contact with the edge of the metamaterial changes the geometry of the material's structure, and therefore how the material responds to stress at the edge. But topological protection ensures that the inside of the metamaterial remains damage free.

This material could one day be used to build cars or rocket launch systems. In cars, the material could help absorb impacts from a crash. "When you're driving a car, you want the car to be stiff and to support a load," Mao said. "During a collision, you want components to become softer to absorb the energy from the collision and protect the passenger in the car."

The researchers also suggest the material could be used to make bicycle tires that can self-adjust to the ride more easily on soft surfaces such as sand, or to make damage-resistant, reusable rockets.

This story is adapted from material from the University of Michigan, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.