Acoustically powered bubble microactuator for robotic manipulation

Oscillating microbubbles generate mean flow, which is known as acoustic streaming (AS), that results in counteracting thrust applied to the bubble. Unlike spherical bubbles, entrapped bubbles with several gas-fluid interfaces can generate directed AS, hence, spatiotemporally controlled thrust. Formulating a fluid-structure interaction model capturing multiple continuous interfaces enables the prediction of the bubble’s natural frequencies (NFs). This calculation is critical because exciting the bubble at the NFs optimizes the performance. We developed a microscopic actuator comprising an entrapped bubble with three interfaces. Its geometry was carefully designed such that at each NF one opening is predominantly deformed, resembling the first vibration mode of a membrane. We used numerical simulations to estimate the generated thrust (i.e., magnitude and direction) due to an impinging planar pressure wave as a function of the excitation frequency. To validate the methodology, a microactuator is 3D printed atop an ultraflexible microcantilever beam using two-photon lithography. Leveraging the principle of superposition, we applied an acoustic field in the form of a combination of three harmonic terms to have the actuator follow complex trajectories with high accuracy.