Twists and Turns of Orbiting and Spinning Metallic Microparticles Powered by Megahertz Ultrasound

Micromotors powered by megahertz ultrasound, first reported about 5 years ago, have lately been considered a promising platform for a wide range of microscale applications, yet we are only at the early stage of understanding their operating mechanisms. Through carefully designed experiments, and by comparing the results to acoustic theories, we present here an in-depth study of the behaviors of particles activated by ultrasound, especially their in-plane orbiting and spinning dynamics. Experiments suggest that metallic microrods orbit in tight circles near the resonance ultrasound frequency, likely driven by localized acoustic streaming due to slightly bent particle shapes. On the other hand, particle spins around their long axes on nodal lines, where phase-mismatched orthogonal sound waves possibly produce a viscous torque. Intriguingly, such a torque spins metal-dielectric Janus microspheres back and forth in an unusual "rocking chair" fashion. Overall, our observations and analysis provide fresh and much needed insights on the interesting particle dynamics in resonating ultrasound and could help with developing more powerful and controllable micromachines with biocompatible energy sources.