Characterizing a Nonreciprocal Active Fluid using 3D Laser Sheet Tomography

Typically, particles in active matter source environmental energy through self-propulsion or from an active bath. Activity can also be "social" --- emerging from effective nonreciprocal interactions between particles. Dusty plasma, a collection of charged microparticles levitated in a plasma, offers an ideal platform to study emergent activity driven by nonreciprocal interactions. In our experiments, micron-sized dust is levitated at the edge of an argon plasma sheath and forms a three-dimensional cloud with thousands of particles. Using high-speed tomographic imaging with a rapidly scanning laser sheet, we simultaneously track thousands of particles in 3D, capturing temporal statistics of single-particle dynamics and collective behavior with ensemble statistics. While most particulate systems jam and dynamics slow as the density is increased, dusty plasma does the opposite. Emergent activity is enhanced when particles are close by, leading to a larger diffusion constant, heavy tails in the displacement distribution, and giant number fluctuations. The transition from passive-to-active reveals an intrinsic property of emergent active fluids, offering new insights into how activity arises in large number non-equilibrium systems.