Acoustically-actuated bubble-powered rotational micro-propellers

•Gear-inspired contactless micro-propellers powered by vibrating bubbles are presented•The propellers consist of symmetric distribution of bubbles that enable fast rotation.•Combined magneto-acoustic fields are used to steer the propellers to specific targets.•The propellers can function like a micro-mixer without any tethered contact to a substrate. Bubble-powered acoustic microsystems span a plethora of applications that range from lab-on-chip diagnostic platforms to targeted interventions as microrobots. Numerous studies strategize this bubble-powered mechanism to generate autonomous self-propulsion of microrobots in response to high frequency sound waves. Herein, we present two micro-propeller designs which contain an axis-symmetric distribution of entrapped bubbles that vibrate to induce fast rotational motion. Our micro-propellers are synthesized using 3D Direct Laser Writing and chemically-functionalized to selectively trap air bubbles at their micro-cavities which function as propulsion units. These rotational acoustic micro-propellers offer a dual advantage of being used as mobile microfluidic mixers, and as autonomous microrobots for targeted manipulation. With regards to targeted manipulation, we demonstrate magneto-acoustic actuation of our first propeller design that can be steered to a desired location to perform rotational motion. Furthermore, our second propeller design comprises of a helical arrangement of bubble-filled cavities which makes it suitable for spatial micro-mixing. Our acoustic propellers can reach speeds of up to 400 RPM (rotations per minute) without requiring any direct contact with a vibrating substrate in contrast to the state-of-the-art rotary acoustic microsystems.