Acoustic resonators as wireless actuators in air for small-scale robots.

While acoustic resonators have been extensively studied in liquids for the realization of untethered microrobots, their implementation in air remains largely unexplored. Here, we revisit the classical principle of Helmholtz resonance and show that 3D printed cavities with sizes ranging from centimeter to micrometer generate spatiotemporally controlled thrust ranging from millinewton to micronewton upon resonance excitation. We provide a comprehensive design framework to efficiently harness acoustic and aerodynamic forces that are validated through analytical modeling, numerical simulations, and experiments. We show robotic navigation of small-scale boats powered by either air-borne or structure-borne sound, highlighting the versatility of the actuation scheme. We present one type of microflier propelled by vertical thrust that weighs only 150 micrograms with a thrust-to-weight ratio of 4.9 and another type of microflier propelled by a rotor that weighs only 184 micrograms and rotates as fast as 13,000 revolutions per minute. The actuation scheme is compatible with even further miniaturization, pushing the boundaries of robotics and aeronautics.