Efficient Propulsion and Hovering of Bubble-Driven Hollow Micromotors underneath an Air–Liquid Interface

Bubble-driven micromotors
have attracted substantial interest due
to their remarkable self-motile and cargo-delivering abilities in
biomedical or environmental applications. Here, we developed a hollow
micromotor that experiences fast self-propulsion underneath an air–liquid
interface by periodic bubble growth and collapse. The collapsing of
a single microbubble induces a ∼1 m·s–1 impulsive jetting flow that instantaneously pushes the micromotor
forward. Unlike previously reported micromotors propelled by the recoiling
of bubbles, cavitation-induced jetting further utilizes the energy
stored in the bubble to propel the micromotor and thus enhances the
energy conversion efficiency by 3 orders of magnitude. Four different
modes of propulsion are, for the first time, identified by quantifying
the dependence of propulsion strength on microbubble size. Meanwhile,
the vertical component of the jetting flow counteracts the buoyancy
of the micromotor-bubble dimer and facilitates counterintuitive hovering
underneath the air–liquid interface. This work not only enriches
the understanding of the propulsion mechanism of bubble-driven micromotors
but also gives insight into the physical aspects of cavitation bubble
dynamics near the air–liquid interface on the microscale.