Ultrasound-driven electronically programmable microbubbles.

Ultrasound-actuated microbubbles have served as efficient microscale engines demonstrating wireless actuation, simplified robotic architectures, and rapid response. Despite recent advances, a limitation of many microbubble technologies is that they use passive capturing of bubbles. The lack of control over bubble placement and size hinders the ability to individually address specified bubbles, a feature that would be important for performing multiple functions. Here, we present an on-chip bubble customization platform that enables active control of individual microbubbles. Instead of trapping bubbles, we purposely generate them via electrolysis on electrode arrays. The bubbles, when actuated by an external ultrasound input, create a variety of programmed flows ranging from streaming due to volumetric oscillation of a single bubble to diverse symmetry-broken directional flows when multiple bubbles interact. The bubble configuration can also be altered by simply increasing the electrolysis voltage on specified electrodes to detach the unneeded bubbles. We find that in this regime, when coupled to acoustic oscillations, we can generate a distinct bubble shooting flow pattern whose flow direction is selectively controlled by the incident ultrasound frequency. Collectively, the active bubble reconfiguration ability unlocks a range of previously inaccessible acoustofluidic functions in a single device with unprecedented controllability.