Energy harvesting based on acoustically oscillating liquid droplets

• Novel actuator for harvesting energy from ambient acoustic noise is developed.
• Effect of acoustic wave on liquid droplet is investigated using high-speed images.
• Energy harvesting is tested using liquid droplet hanging from piezocantilever.
• Piezocantilever output voltage is measured by custom-made electric circuit.
• Maximum power for load (10 Ω) is measured to be about 80 μW.

This paper presents a novel actuator for harvesting energy from ambient acoustic noise using acoustically oscillating droplets. When a liquid droplet sitting on a piezocantilever is excited by an acoustic wave around its natural frequency, its oscillation simultaneously bends the piezocantilever, which generates electric power owing to the piezoelectric effect. The oscillation amplitudes of water droplets with three different sizes (2, 4, and 6 μl) hanging from a solid substrate were first investigated using high-speed images. The results showed that the droplet oscillation amplitude was strongly dependent on the applied frequency and was proportional to the droplet size. The maximum droplet oscillation amplitude occurred at the natural frequency of the droplets. Energy harvesting based on acoustically oscillating droplets was separately tested using a commercial piezocantilever. The oscillation behaviors of water droplets hanging from a flexible piezocantilever were also studied using high-speed images. The bending displacement and generated voltage of the piezocantilever by the acoustically oscillating water droplets were measured with a high-speed camera and digital oscilloscope, respectively, for different droplet sizes and distances between the droplet and piezoactuator. Both the bending displacement and generated voltage were strongly affected by the applied frequency and proportional to the droplet size but were inversely proportional to the distance. The force generated from the acoustically oscillating droplets was measured by using a load cell. The maximum force generated from the acoustically oscillating droplet (4 μl) was about 123.9 μN at the maximum bending displacement (about 1.5 mm). The output voltage and power generated from the piezocantilever actuated by the acoustically oscillating droplets were measured with a custom-made electric circuit (mainly consisting of a voltage rectifier and load) for different droplet sizes. The maximum generated power for the load (10 Ω) was measured to be about 80 μW. As proof of concept, storage capacitor charging tests were conducted for 0.1 and 1 μF capacitors using the acoustically oscillating droplets in three different sizes.