High-concentration nanobubble generation by megasonic cavitation and atomization

Abstract

A new nanobubble generation method was proposed to dramatically increase bubble concentration and size uniformity through mega-hertz ultrasonic cavitation and atomization. The megasonic transducer generated fine droplets, including high-density bubble nuclei, from liquid surfaces into the air via capillary waves and acoustic cavitation. The atomization-assisted ultrasonic method achieved a 100-fold increase in nanobubble density with the highest 2.25×109/ml through 1.7 MHz ultrasound, which has not been reported in the literature. Our results also showed that the nanobubble concentration increased with increasing ultrasound power and sonication time and reached maximum concentration, which occurred earlier at higher ultrasonic power. The average bubble diameter increased slightly with increasing frequency, and the dependence of ultrasound power and sonication time on mean diameter was small. The atomized droplets dramatically increased the density and size uniformity of bulk nanobubbles in water; hence, overcoming the inherent limitation of low bubble density by mega-hertz acoustic cavitation.