The non-negligible role of opening size in acoustic streaming thrust of entrapped microbubbles

Ultrasound-actuated microrobots that incorporate entrapped microbubbles rely on acoustic streaming thrust for propulsion. While bubble size and resonance have been extensively studied, the role of opening size remains less understood. In this study, we numerically investigate how the opening size affects both streaming flow and the resulting thrust. Finite element analysis (FEA) reveals that although streaming velocity increases with oscillation amplitude and opening size, the generated thrust does not always scale proportionally. Counterintuitively, microbubbles with larger openings are more likely to produce greater thrust than those with smaller openings under identical acoustic excitation, despite generating weaker streaming flows. This phenomenon is experimentally confirmed using a microcantilever-based thrust measurement system. Our study provides key insights into the mechanics of microbubble-driven propulsion and highlight the the critical role of opening size in acoustic streaming thrust, offering valuable guidance for the design of next-generation microbubble-based acoustic microrobots for various biomedical applications.