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

Abstract

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.

Leveraging collective effects in externally driven colloidal suspensions: experiments and simulations

“…Acoustic levitation can also lead to the spinning of rods and spheres about an axis parallel to the levitation plane [99,106]. The physical mechanisms behind this behavior are still debated: Wang et al [99] claim that the spinning is related to the helical elastic surface waves (called Rayleigh waves) on the colloids generated by the acoustic field, while, according to Zhou et al [106], the spinning is due to acoustic waves with different phases traveling in the plane that generate a viscous torque on the particles. Rods have also been observed to orbit in tight circles in the levitation plane [99,106].…”

“…The physical mechanisms behind this behavior are still debated: Wang et al [99] claim that the spinning is related to the helical elastic surface waves (called Rayleigh waves) on the colloids generated by the acoustic field, while, according to Zhou et al [106], the spinning is due to acoustic waves with different phases traveling in the plane that generate a viscous torque on the particles. Rods have also been observed to orbit in tight circles in the levitation plane [99,106]. According to [106], these orbiting trajectories come from the torque generated by the steady flow from acoustic streaming on the slightly bent shape of the particles about the rotation axis.…”

“…Rods have also been observed to orbit in tight circles in the levitation plane [99,106]. According to [106], these orbiting trajectories come from the torque generated by the steady flow from acoustic streaming on the slightly bent shape of the particles about the rotation axis.…”

“…At the collective level, suspensions of acoustically powered colloids exhibit interesting behaviours due to acoustic radiation, hydrodynamic interactions, or a competition between these two effects. Self assembly of rods and spheres into large chains of particles have been experimentally observed [99,106]. These chains are assumed to form due to secondary acoustic radiation forces (i.e.…”

“…These chains are assumed to form due to secondary acoustic radiation forces (i.e. Bjerknes forces) that are attractive in the levitation plane [108,109,106]. When the particles are spinning, the chains form a large vortex that advects particles at large speeds due to hydrodynamic coupling [99,106].…”