Nonreciprocity and multibody interactions in acoustically levitated particle systems: A three-body problem

Pairwise nonreciprocal interactions are known to drive nonequilibrium collective phenomena. However, there also exist interactions that are reciprocal for a pair of objects but break reciprocity through multibody effects. In this case, nonreciprocity emerges spontaneously, without having to use special objects or tailor the interactions. Here, we demonstrate this with three identical spheres acoustically levitated in air that become excited into novel dynamical states through nonreciprocal multibody interactions induced by sound. This three-sphere system allows detailed interrogation of the competition between wave scattering and viscous microstreaming in generating self-organized, dynamic steady states that exhibit collective limit cycles and self-propulsion. Crucially, energy from the acoustic field is redirected to power these dynamics when the configuration of the three particles breaks inversion symmetry. These results introduce a minimal model system for emergent activity and open up new possibilities for self-assembly, where multibody interactions not only determine the resulting structure but also drive spontaneously emerging dynamics.