Bistability and mode cycling in nonreciprocal robotic active solids

Active solids, unlike passive ones, can sustain internal waves and motion through continuous energy input, enabling collective modes absent in equilibrium materials. A central question is how interaction symmetry and boundary conditions shape these emergent modes. To address this, we build active solids from bio-inspired chiral spinning robots that interact at the air–water interface via hydrodynamic and magnetic forces. We find that within confined lattices, nonreciprocal interactions drive bistable dynamics that alternate between stationary and cycling states. A minimal chiral spinner model reproduces these transitions and reveals a Hopf-like bifurcation governing the switch between oscillatory and quiescent phases. Normal-mode analysis further uncovers handed cyclic trajectories in mode space, indicating presence of odd elasticity. Together, these results demonstrate how nonreciprocity and boundary conditions govern collective dynamics, establishing robotic active matter as a controllable platform for probing out-of-equilibrium solid mechanics.