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

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.

Automated summary

The study revealed that the shape of micromotors has a significant impact on propulsion speed at low fuel concentrations, with the concave Pt-coated micromotor moving slower and the bubble growth being slower on its surface compared to the convex Pt-coated micromotor. In the multi-bubble propulsion mode at high fuel concentrations, increasing fuel concentration did not enhance maximum propulsion speed, suggesting a more is less phenomenon in the interaction among bubbles.