Nonreciprocal multi-body interactions activate liquid state of acoustically levitated particle ensembles

Abstract

Nonreciprocal forces are often a consequence of asymmetry in the properties of the interacting objects. However. even if all objects are identical and isotropic, and the pairwise interactions between two objects are completely reciprocal, nonreciprocal forces can still appear when an arrangement of many objects breaks configurational symmetry in the presence of non-pairwise, multi-body interactions. Here we demonstrate that such emergent nonreciprocity can activate a particle ensemble to behave like a liquid, albeit with unique traits. In our experiments, passive microspheres are acoustically levitated in air, where they form a freely floating monolayer containing up to a couple hundred particles and collectively behave like a two-dimensional liquid droplet. The particles interact via nonreciprocal multi-body forces that arise from the combination of acoustic scattering and sound-induced viscous microstreaming. We find that these forces drive superdiffusive particle motion with non-Gaussian tails in the particles' speed distribution. Using probes that reach laterally into the levitation plane, we perform liquid pendant drop and pinch-off experiments. Compared to ordinary liquids, the droplets are found to have a kinematic viscosity similar to that of water, but in combination with an extremely low interfacial tension. The pinching-off is driven by nonreciprocity-induced active fluctuations and exhibits the self-similar double-cone neck profile seen also in liquid nanojets close to rupture, however here characterized by power law behavior with a scaling exponent that is anomalously small.