Engineering news
Engineers at The University of Texas at Austin and the AMOLF Institute in the Netherlands have developed a mechanical metamaterial that transfers motion in one direction while blocking it in the other.
The nonreciprocal material can be thought of as a mechanical one-way shield that blocks energy from coming in, but easily transmits it going out the other side. Metamaterials are synthetic materials with properties that cannot be found in nature.
Breaking the symmetry of motion may enable greater control on mechanical systems and improved efficiency. The nonreciprocal metamaterial can potentially be used to realise different types of mechanical devices. For example, actuators and other devices that could improve energy absorption, conversion and harvesting, soft robotics and prosthetics.
The discovery lies in the ability to overcome reciprocity, a fundamental principle governing many physical systems, which ensures that we get the same response when we push an arbitrary structure from opposite directions. This principle governs how signals of various forms travel in space and explains why, if we can send a radio or an acoustic signal, we can also receive it. In mechanics, reciprocity implies that motion through an object is transmitted symmetrically. If by pushing on side A we move side B by a certain amount, we can expect the same motion at side A when pushing B.
Andrea Alu, a professor in the Cockrell School of Engineering, said: “The mechanical metamaterials we created provide 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.”
The researchers first created a rubber-made, centimeter-scale metamaterial with a specifically tailored fishbone skeleton design. They tailored its design to meet the main conditions to break reciprocity, namely asymmetry and a response that is not linearly proportional to the exerted force.
Corentin Coulais, an AMOLF researcher, said: “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 the conditions for reciprocity, and we can achieve a very large nonreciprocal response.”
The material’s structure is a lattice of squares and diamonds that is completely homogeneous throughout the sample, like an ordinary material. However, each unit of the lattice is slightly tilted in a certain way, and this subtle difference dramatically controls the way the metamaterial responds to external stimuli.
Dimitrios Sounas, a researcher at Cockrell School of Engineering, said: “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.”
When the researchers apply a force on the soft side of the metamaterial, it easily induces rotations of the squares and diamonds within the structure, but only in the near vicinity of the pressure point, and the effect on the other side is small. Conversely, when they apply the same force on the rigid side, the motion propagates and is amplified throughout the material, with a large effect at the other side. As a result, pushing from the left or from the right results in very different responses, yielding a large nonreciprocity even for small applied forces.
The team is now working on optimising the metamaterials and developing devices for applications in soft robotics, prosthetics and energy harvesting.
The study is described in a paper published in the journal
Nature.