Non-reciprocal coupling and collective actuation in the presence of a polarization field.

Active solids consist of elastically coupled out-of-equilibrium units performing work. Recently, it was shown that the microscopic non-reciprocal coupling between displacements and polar forces give rise to a wide variety of collective actuation at large scale. Such phenomena are central to deciphering the complex physics behind biological systems ranging from confined cell monolayers to dense bacterial suspensions and bio-films. Remarkably, such living systems also have the ability to respond to various types of environmental cues and can polarize towards or away from these signals, e.g. by chemotaxis or galvanotaxis. Yet, the effect of an external field on the collective dynamics of active solids remains, until today, largely unexplored. Here, we combine model experiments, numerical simulations, and theoretical analysis to reveal how an external field affects the dynamics of active elastic systems. We find that gravity is suitable to polarize our model active solids, and crucially, polarization decreases the activity threshold for collective actuation. Our findings may provide new mechanisms for oscillatory dynamics and regulation in biological tissues.