Innovative Nonreciprocal Mechanical Metamaterials Could Add Value to Newer Mechanical Devices

This is an artist's rendering of mechanical metamaterials. CREDIT: Cockrell School of Engineering.

The world’s first mechanical metamaterials with the ability to effortlessly transfer motion in one direction and to block it in the other have been developed by engineers and researchers at The University of Texas at Austin and the AMOLF institute, Netherlands.

The findings of the research have been reported in a paper published in the February 13th issue of the journal Nature. The material can be considered similar to a mechanical one-way shield that prevents energy from coming in but effortlessly transmits it to the other side.

Metamaterials - synthetic materials with characteristics not found in nature - have been used to develop the first nonreciprocal mechanical materials.

Mechanical systems can be efficiently controlled by breaking the symmetry of motion. New types of mechanical devices - such as prosthetics, soft robotics, actuators (i.e. machine components important for controlling or moving a mechanism), and other devices with the potential to enhance energy absorption, conversion, and harvesting - can be developed by potentially using nonreciprocal metamaterials.

The success of the researchers depends on the potential to overcome reciprocity (a fundamental principle in various physical systems), which ensures that the same response is obtained while pushing an arbitrary structure from opposite directions.

This principle controls the manner in which signals of different forms travel in space and gives a clear picture of the reason behind the ability of sending a radio or an acoustic signal, as well as receiving it. In the field of mechanics, reciprocity signifies the symmetric transmission of motion through an object; if side B is moved by a specific amount by pushing on side A, the same motion can be expected at side A when side B is pushed.

The mechanical metamaterials we created provide new elements in the palette that material scientists can use in order to design mechanical structures. This can be of extreme interest for applications in which it is desirable to break the natural symmetry with which the displacement of molecules travels in the microstructure of a material.

Andrea Alu, Professor, Cockrell School of Engineering

In the last couple years, Alu has worked in collaboration with Dimitrios Sounas, Cockrell School research scientist, as well as other members of the research group to make astonishing discoveries in the field of nonreciprocal devices for acoustics and electromagnetics.

The discoveries include the development of novel nonreciprocal devices for radio waves, sound waves, and light. On a visit to the AMOLF institute, Netherlands, the research team started a constructive cooperation and close interaction with Corentin Coulais, an AMOLF researcher working of late on the development of mechanical metamaterials, which resulted in the research team’s success.

Initially, the researchers developed a centimeter-scale, rubber-made metamaterial possessing a uniquely tailored fishbone skeleton design. The design was tailored to satisfy the main conditions to overcome reciprocity, i.e. asymmetry and a response not linearly proportional to the exerted force.

This structure provided us inspiration for the design of a second metamaterial, with unusually strong nonreciprocal properties. By substituting the simple geometrical elements of the fishbone metamaterial with a more intricate architecture made of connected squares and diamonds, we found that we can break very strongly the conditions for reciprocity, and we can achieve a very large nonreciprocal response.

Corentin Coulais, Researcher, AMOLF

The structure of material includes a lattice of diamonds and squares, where the lattice is thoroughly homogeneous throughout the sample, like an ordinary material. Yet, each unit of the lattice is slightly tilted in a particular way, and this difference hugely governs the manner in which the metamaterial responds to external stimuli.

The metamaterial as a whole reacts asymmetrically, with one very rigid side and one very soft side. The relation between the unit asymmetry and the soft side location can be predicted by a very generic mathematical framework called topology. Here, when the architectural units lean left, the right side of the metamaterial will be very soft, and vice-versa.

Dimitrious Sounas, Research Scientist, Cockrell School of Engineering

Upon applying a force on its soft side, the metamaterial could easily induce rotations of the diamonds and squares within the structure, however only in the near vicinity of the pressure point. Yet, the effect on the other side is meager.

By contrast, when the same force is applied on the rigid side, the motion gets propagated and gets amplified all through the material, resulting in a large effect at the other side. Consequently, varied responses were evident while pushing from left and from right, resulting in a large nonreciprocity even in the case of small applied forces.

The team is working toward leveraging the topological mechanical metamaterials for different applications, optimizing metamaterials, as well as developing different devices for applications in prosthetics, soft robotics, and energy harvesting.

The Air Force Office of Scientific Research, the Simons Foundation, the Office of Naval Research, the Netherlands Organization for Scientific Research, and the National Science Foundation funded the research.

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