Self-Assembly of Acoustically Levitated Granular Crystals

We acoustically levitate sub-millimeter particles to visually examine the structural rearrangements of self-assembled granular crystals. By tuning the interplay between acoustic and electrostatic forces, we are able to generate structures ranging from compact particle rafts with long-range crystalline order, to Wigner crystals where the electrostatic repulsion between particles enforces a minimum equilibrium distance. In particular, we observe and characterize the transitions between degenerate ground states in the free energy landscape of compact cluster configurations. Our results provide a macroscopic experimental model for self-assembly, as well as a means to probe tribocharging in assemblies of insulating grains, which may shed light on the formation of planetesimals in protoplanetary disks.