Efficient propulsion and hovering of bubble-driven hollow micromotors underneath an air-liquid interface

The autonomous motion of artificial micromotors has attracted great attention recently owing to their applications in biomedical and environmental areas. In this study, we develop a new type of hollow self-motile Janus micromotor that moves just underneath the air-liquid interface driven by microbubble cavitation. It is the first time to our best knowledge to report the propulsion mechanism of the cavitation-induced-jetting for micromotor. In contrast to previous studies that only focused on the direct momentum exchange between a Janus micromotor and microbubbles, we unveil the significant contribution of the hydrodynamic effect during the bubble collapse stage. Four different modes of propulsion are identified by quantifying the dependence of propulsion strength on microbubble size. Different from common solid Janus micromotors, our hollow micromotors manifest instinct propulsion behavior underneath the air-liquid interface. In particular, the mechanism of the vertical dynamic hovering can keep a micro-object whose density is lower than water moving beneath the air-liquid interface. We show that the cavitation-induced jetting better utilizes the energy stored in the bubble to propel the micromotor, and thus enhances the energy conversion efficiency by three orders of magnitude.