Controlling elasticity in levitated active hinges

The assembly of millimeter-sized structures remains an outstanding difficulty, particularly those which can be programmed for precise actuation. We use acoustic levitation to address this issue, by which we can assemble sub-millimeter granular clusters with tunable interactions. Acoustic scattering between levitated particles generates attractive forces which are strongly shape-dependent. In particular, cube-shaped particles preferentially come into contact by sharing a single edge and exhibit a hinge-like motion around this connection. We inject energy into the elastic modes of this granular hinge by rapidly switching the acoustic field off and on, tuning the amplitude of these modes. We measure how the elasticity of this nonequilibrium hinge depends on the initial injection of energy and show that acoustic forces act to drive the soft modes into synchronization with the translational motion of the cluster. Our work opens the door to robust, controllable elasticity in a self-assembled, active granular system.