Distinct dynamics of self-propelled bowl-shaped micromotors caused by shape effect: Concave vs convex

Abstract

Although artificial micromotors with unconventional shapes are emerging as a powerful tool in various applications, little research has been undertaken to clarify their propulsion mechanism, especially how the shape effect alters the bubble dynamics and hydrodynamic flows. In this study, we fabricated two types of bowl-shaped micromotors to investigate the distinct dynamics due to the shape effect of concave and convex surfaces, by coating a platinum (Pt) layer on either the concave surface or the convex surface of the micromotor. In the single-bubble propulsion mode at low fuel concentration, the concave-surface-Pt-coated micromotor moved unexpectedly slower than the convex-surface-Pt-coated micromotor, and the bubble growth on the concave surface was also much slower than that on the convex surface. It was elucidated that the confinement effect of the concave surface hindered fuel replenishment and thus the catalytic reaction. We further introduced the Kelvin impulse to explain why the concave shape eventually weakened the propulsion from hydrodynamic jet flows caused by bubble collapse. In the multi-bubble propulsion mode at high fuel concentration, the interaction among bubbles rendered a “more is less” phenomenon—increase in the fuel concentration did not enhance the maximum instantaneous propulsion speed. These findings inspire the development of new manipulation strategies utilizing the unconventional shape effect in micromotors.