Launching by cavitation.

Cavitation, characterized by formation of vapor bubbles in a low-pressure or high-temperature region of a liquid, is often destructive, but it can be harnessed for actuators and robots. We exploit cavitation to accumulate substantial energy in superheated liquids by suppressing its immediate release until reaching a stability limit. The energetic, unstable bubbles collapse violently, producing a burst of high power and force that initiates motion. Notably, a millimeter-scale device launched by cavitation can jump to a height of 1.5 meters—reaching a 12 meters per second (m/s) peak velocity, a 7.14 × 104 m/s2 acceleration, and a 0.64% energy efficiency—and can also swim on water at 12 centimeters per second. Cavitation-based launching works with a broad range of device materials, liquid media, stimuli, and operational environments. Editor's summary: Rapid motions, such as jumping, require the fast conversion of a large amount of potential energy into kinetic energy. For a robotic system, this can tax the limits of the onboard power supply. Wang et al. designed a method to launch a robot that uses focused near-infrared heating, ultrasound, or electrical sparks to remotely initiate the growth and catastrophic collapse of cavitation bubbles. Key to the design is the suppression of bubble release until a stability limit is reached, allowing the energy contained therein to be harnessed upon the bubbles' violent collapse. The authors show that, in addition to jumping, the same approach can be used to power the swimming of a robot. They also demonstrate that this concept works for a range of materials, liquids, and operating environments. —Marc S. Lavine [ABSTRACT FROM AUTHOR]