A novel actuator for energy harvesting using an acoustically oscillating liquid droplet

This paper presents a novel actuator for energy harvesting 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, it oscillates and simultaneously bends the piezocantilever by the reaction of the droplet oscillation, resulting in electric power generation owing to the piezoelectric effect. The oscillation amplitudes of three different sized water droplets actuated by acoustic waves in a wide range of frequencies generated from a cylindrical piezoactuator are firstly investigated using a high speed camera. The results show that the droplet oscillation amplitude is strongly dependent on the applied frequency and proportional to the droplet size. And the maximum droplet oscillation amplitude occurs at the droplet natural frequency. The energy harvesting based on an acoustically oscillating droplet is separately tested using a commercial piezocantilever. The bending displacement and generating voltage of the piezocantilever by an acoustically oscillating water droplet with three different sizes in different distances between the droplet and piezoactuator are measured by using a high speed camera and digital oscilloscope, respectively. The both bending displacement and generating voltage are also highly affected by the applied frequency and proportional to the droplet size but inversely proportional to the distance. The force generated from an acoustically oscillating droplet is also measured using a load cell. The maximum force generated from the acoustically oscillating droplet is about 120 μN at the maximum bending displacement (about 1.5 mm). Finally, as the concept proof, the output voltage and power generated from the piezocantilever actuated by an acoustically oscillating droplet (6 μl) are measured using a custom-made electric circuit mainly consisting of voltage rectifier and load. The maximum generated power for the load- (470 kΩ) is measured about 72.2 μW.