U-M Scientists Design New Metamaterial that can Switch Between Soft and Hard

When a material is created, you cannot change whether that material is soft or hard. However, a group of scientists at University of Michigan have developed a new method to design a "​metamaterial" that can switch between being soft and hard without altering or damaging the material.

Metamaterials are man-made materials, and they get their properties - in this case, whether a material is soft or hard - from the way the material is constructed rather than the material that constructs it. This enables scientist to control the structure of a metamaterial in order to make the material display a certain property.

In the study, published in the Nature Communications journal, the U-M scientists discovered a method to compose a metamaterial that can be easily controlled to increase the stiffness of the material’s surface by orders of magnitude - the difference between steel and rubber.

As these properties are "topologically protected," considering that the properties of the material come from its total structure, they can be easily maintained although the material shifts repeatedly between its soft and hard states.

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, Assistant Professor of Physics, University of Michigan

For instance, a dental filling cannot be changed once the filling has been set by the dentist without causing stress, either by grinding or drilling, to the original filling. In the same way, a guitar string cannot be tightened without giving stress on the string itself, said Mao.

According to Mao, the way an object comes in contact with the edge of the metamaterial changes the geometry of the structure of the material, and so how the material react to stress at the edge. However, the metamaterial's topological protection enables the inside of the metamaterial to remain damage-free.

One day this material could be used to build rocket launch systems or cars. In the case of cars, the material could help absorb impacts from an accident.

When you're driving a car, you want the car to be stiff and to support a load. During a collision, you want components to become softer to absorb the energy from the collision and protect the passenger in the car.

Xiaoming Mao, Assistant Professor of Physics, University of Michigan

The researchers also suggest the material could be used for making damage-resistant, reusable rockets or in bicycle tires that self-adjust to ride more easily on soft surfaces like sand.

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