Guided transition waves in multistable mechanical metamaterials

Significance Mimicking material-level phenomena using macroscopically architected materials has gained popularity and enabled novel engineering applications such as photonic, acoustic, mechanical, and topological metamaterials. An interesting microstructural phenomenon observed in phase-transforming materials is the dissipative motion of topological defects such as phase and domain boundaries. With a few one-dimensional exceptions, structural analogs of dynamic phase-transforming materials are still rare, owing to their complicating strong nonlinearity. Through experiments, models, and simulations, we demonstrate a concept for tailoring propagating transition fronts in periodic structures in arbitrary dimensions. This significantly increases the design space of metamaterial performance and functionality and finds application in programming soft robotic locomotion, in controlling energy absorption (or release), and in mechanical logic devices. Transition fronts, moving through solids and fluids in the form of propagating domain or phase boundaries, have recently been mimicked at the structural level in bistable architectures. What has been limited to simple one-dimensional (1D) examples is here cast into a blueprint for higher dimensions, demonstrated through 2D experiments and described by a continuum mechanical model that draws inspiration from phase transition theory in crystalline solids. Unlike materials, the presented structural analogs admit precise control of the transition wave’s direction, shape, and velocity through spatially tailoring the underlying periodic network architecture (locally varying the shape or stiffness of the fundamental building blocks, and exploiting interactions of transition fronts with lattice defects such as point defects and free surfaces). The outcome is a predictable and programmable strongly nonlinear metamaterial motion with potential for, for example, propulsion in soft robotics, morphing surfaces, reconfigurable devices, mechanical logic, and controlled energy absorption.