Acoustically levitated lock and key grains

The self-assembly of materials with complex structure requires mechanisms for programmable, specific bonds between subunits. Such mechanisms have been achieved in micrometer size systems, using colloids bonded with complementary DNA strands or shape-dependent depletion forces. However, equivalent schemes for out-of-equilibrium directed assembly are just beginning to be explored. Here we show that acoustic levitation offers one possibility for the generation of shape-dependent, specific bonds between pairs of millimeter scale particles. We levitate particles in an ultrasonic standing wave, allowing for substrate-free assembly. Secondary scattering generates shape-dependent attractive forces between particles, while driving the acoustic trap above its resonance frequency produces active fluctuations that mimic an effective temperature. We 3D print planar particles, and show that the local curvature of their binding sites tunes the energy landscape along the particle perimeter, in turn controlling the selectivity and bound-state lifetime for attaching a matching particle to the binding site. We show that these principles can be used to design and assemble particles into complex structures.