Selective and Collective Actuation in Active Solids

Active solids consist of elastically coupled out-of-equilibrium units performing work. They are central to autonomous processes, such as locomotion, self-oscillations and rectification, in biological systems, designer materials and robotics. Yet, the feedback mechanism between elastic and active forces, and the possible emergence of collective behaviours in a mechanically stable elastic solid remains elusive. Making use of centimetric self-propelled particles, we construct the first experimental model system of active elastic material, and study its emerging behaviors in various mechanical conditions. We find that active units acting at the nodes of an elastic structure spontaneously organize and actuate selectively a few modes of the structure. Crucially, they are not necessarily the lowest energy ones. Combining experiments with the numerical and theoretical analysis of an agents model, we unveil the bifurcation scenario and the selection mechanism by which the collective actuation takes place. Our findings may provide a new mechanism for oscillatory dynamics in biological tissues, and opens the path toward the design of meta-materials with bona fide autonomy.