Reentrant Transition to Collective Actuation in Active Solids with a Polarizing Field

Collective actuation in active solids-the spontaneous coherent excitation of a few vibrational modes-emerges from feedback between structural deformations and the orientation of active forces. It is an excellent candidate as a basic mechanism for oscillatory dynamics and regulation in dense living systems, and better control over its onset would open new avenues in the life sciences. Combining model experiments, simulations, and theory, we study the dynamics of such an active solid in the presence of an external field that polarizes the active forces. The experiments reveal a novel oscillatory regime absent at zero field. The theoretical analysis of a single agent demonstrates that the small field oscillations and the large field ones can be mapped onto the bounded and unbounded phase dynamics of a nonlinear pendulum. In the many-agents case, the transition to collective actuation is promoted at low field, leading to a reentrant transition.