Base states and transitions of an acoustically levitated cluster of anisotropic particles

Anisotropic particles are important building blocks for the self-assembly of complex structures. However, little is known about the energy landscape of small clusters made of anisotropic particles and even less for clusters formed through underdamped motion in the presence of nonthermal fluctuations. Acoustic levitation, a method of suspending sub-millimeter objects against the force of gravity in air, is an ideal tool to investigate assembly under these conditions. We levitate low-aspect ratio rods of diameter 40-50μm in the nodal plane of an acoustic standing wave, where geometry-dependent secondary scattering forces cause anisotropic attraction and assembly rods into dense clusters. By tuning the driving frequency of the acoustic trap off-resonance, we introduce stochastic nonthermal forces arising from hydrodynamic coupling in the system. Activating and cooling the rod cluster with such forces allows us to study the cluster's underdamped structural transformations, which we compare with molecular dynamics simulations of thermally formed anisotropic particle clusters.