Cleaning Surfaces with Bubbles with Sub-Resonance Acoustic Waves

The use of bubbles in an aqueous medium to clean biological surfaces holds promising potential for developing sustainable methods to clean agricultural produce. Bubbles with a diameter of 1.2 mm exhibit a resonant frequency of approximately 5300 Hz. However, one of the low sub-resonant frequencies near 90 Hz induces a distinctive swaying motion when suspended underwater. This swaying motion at sub-resonance facilitates a unique sliding behavior as bubbles ascend inclined surfaces, presumably enhancing shear stress and cleaning efficacy. Additionally, the sub-resonant frequency causes bubbles to deviate from a linear path, curving away from the sound source and colliding with surfaces with a greater perpendicular component of speed, increasing shear force and bounce height, further improving the cleaning process. The observed sub-resonance effects are confirmed to be independent of the boundary effect, as demonstrated by replicating the same sub-resonance in both large and small tanks. This research sheds light on the potential of acoustically manipulated bubbles as an effective, non-chemical cleaning technique for agricultural produce and other surfaces.