Electrostatic ground states of precisely charged, acoustically levitated granular matter

Acoustic levitation serves as a versatile platform for the assembly and manipulation of granular particles. Granular microspheres on the order of several hundred microns in diameter typically assemble into two-dimensional, close-packed clusters due to attractive forces that arise from acoustic scattering. We propose a mechanism that takes advantage of inductive charging to force the levitated microspheres into stable configurations where the interparticle distances are several times the particle diameter. Specifically, we show that our electrostatic induction charging technique can be used to precisely define the charge-state of individual conductive microspheres. This enables us to overcome the attractive acoustic forces and control the stability of levitated microspheres with repulsive Coulomb forces and electrostatic dipole forces. We observe large interparticle distances between multiple stably levitated like-charged microspheres. Our results constitute the building block for granular scale Coulomb crystals—a system with complex dynamics and rich physics.