square[source]  Pushing and pulling on a material’s surface in a steady, periodic way is a common test for characterizing that material’s mechanical properties. Simple materials typically exhibit a single steady state response to a given way of forcing. We proposed a metamaterial made of mechanically bistable elements, which responds with a wide variety of distinct steady state cycles in response to just a single, persistently applied periodic force. This metamaterial could be a step towards building materials that exhibit dynamic forms of memory in their response to periodic driving.

Each unit cell in the metamaterial spontaneously adopts one of two minimum energy deformation modes. As a result, different regions within the material settle into one or the other stable mechanical phases, analogous to competing positive and negative domains in a ferromagnet. The boundary force pattern selects the preferred mechanical phase along a given portion of the boundary. The shape and speed of the internal divisions between mechanical phases can be tuned based on the stiffness of the internal components. Since a wide variety of internal configurations can be compatible with the same chosen phase pattern selected by an applied force along the materials edge, the system is found to settle into many different steady state cycles when the applied force oscillates. The system has a large degree of flexibility and potential for programmability. The observed responses could be tailored by changing the number and distribution of forcing segments along the boundary, testing different system shapes with distinct boundary symmetries, or by forcing different portions of the boundary out of phase or with different amplitudes.



Topologically protected steady cycles in an ice-like mechanical metamaterial
C. Merrigan, C. Nisoli, and Y. Shokef
Physical Review Research 3, 023174 (2021)