Manipulating Complex Granular Structures Through Combined Acoustic Levitation and Electrostatic Modulation

Acoustic levitation provides a unique platform for the self–assembly of granular matter without the constraints of gravity or container walls. We have developed a technique that combines acoustic levitation with controlled electrostatic charging to assemble, transform, and activate complex, many–bodied granular structures. In the presence of an acoustic field alone, particles aggregate into densely packed or "collapsed" formations due to attractive forces arising from acoustic scattering. However, introducing long–range electrostatic repulsion enables some particles to overcome these attractions, resulting in "expanded" and "mixed" structures. For small particle numbers, electrostatic forces can dominate entirely, leading to fully expanded structures that closely match simulated electrostatic ground states. For larger ensembles, the interplay of acoustic and electrostatic forces typically yields mixed structures with a dense core surrounded by "satellite" particles—a configuration unattainable with either interaction alone. We observe and investigate the synchronized oscillations and coupled motions between the dense core and the satellite particles. Additionally, we can neutralize the electrostatic charges of the expanded and mixed structures with soft X–rays, which triggers their collapse and induces rotation. Our approach opens the door to precise control over the configurations and dynamics of complex granular assemblies.