Density Asymmetry Driven Propulsion of Ultrasound-Powered Janus Micromotors

The ability of acoustically propelled micro and nanoscale motors to perform diverse tasks while moving in solutions can open up new applications in diverse fields such as medicine, biotechnology, and materials science. However, the current understanding of the underlying propulsion mechanisms of ultrasound-driven structures is limited for translating their motion and operation to practical applications. Here, the behavior of Janus microparticles displaying acoustically driven propulsion is demonstrated. A new approach to harness the acoustically-induced vibration and oscillation of a density asymmetric Janus microstructure into translational motion is presented, based on fixing the micromotor orientation with an external magnetic field. Such acoustic propulsion of Janus microparticles is realized through a judicious material selection based primarily on density and asymmetry considerations. Experimental data and theoretical models indicate that the density asymmetry provides an acoustic propulsive force for translational motion. The Janus structure presented here is also able to propel using chemical and magnetic actuations, paving the way for different hybrid nanovehicles. The new approach to harvest acoustic energy leads to a robust motile platform and expands the horizons of ultrasound-propelled micro/nanomotors, offering new possibilities for their design and applications.