Non-reciprocal interactions in the pairing dynamics of a dusty plasma

Many-body systems with effective interactions that violate Newton’s third law are often found in biological systems, such as bird flocks. In contrast to the complex conditions in biology, dusty plasma, a non-equilibrium system, provides a simpler platform for studying non-reciprocal forces in many-body systems. When levitated at the edge of an argon plasma sheath, micron-sized dust particles can form three-dimensional structures when the confinement is sufficiently strong. Our experiments utilize a permanent magnetic field to enhance confinement, and vertical pairs of particles form due to strong, non-reciprocal interactions from an ion wake in the plasma sheath. Importantly, the 3D tomographic imaging of our system using a rapidly scanning laser sheet allows us to simultaneously characterize the individual position and velocity of thousands of particles and the collective behavior of the whole system, which is important since we find there are gradients in particle temperature and local density within the sheath. The distribution of particle kinetic energy and density directly affects the formation of vertical pairs. The above phenomenon can be revealed by comparing simulation and experimental results. The current analysis of particle pairs can help us bridge the gap between the characteristics of individual particles and the statistical properties of the entire system.