Multimodal bubble microrobot near free liquid surface

Abstract The development of multifunctional and robust swimming microrobots working at the free air-liquid interface has encountered challenge as new manipulation strategy are needed to overcome the complicated interfacial restrictions. Here, we show flexible but reliable mechanisms to achieve a remote-control bubble microrobot with multiple working modes and high maneuverability by the assistance of a soft air-liquid interface. This bubble microrobot is developed from a hollow Janus microsphere regulated by a magnetic field, which can implement switchable working modes like pusher, gripper, anchor and sweeper. The cavitation of the microbubble and the accompanying directional jet flow play a key role for functioning in these working modes, which is analogues to "bubble tentacle". In particular, we find a novel speed modulation method for the bubble microrobot that does not need to change the fuel concentration. Our findings demonstrate a substantial advance of the bubble microrobot specifically working at the air-liquid interface. And the results depict the nonintuitive mechanisms, such as bubble dynamics confined by a free surface, hydrodynamic jetting and surface capillary wave induced by bubble collapse, and bubble particle interaction, which offer helpful insights for developing swimming microrobots working in complicated environments.