Ultrasound-enhanced mass transfer during the growth and dissolution of surface gas bubbles

Proper understanding and control of the mass transfer capability of acoustically-driven gas bubbles is crucial for the safety of biomedical applications and the efficiency of many electrochemical processes. Here, we quantify experimentally the effect of ultrasound on the rate of dissolution and growth of a gas bubble in contact with a solid surface, focusing on the dynamics of the bubble radius on the diffusive time scale. Significant degrees of super- or undersaturation of the surrounding carbonated water ensure that acoustic microstreaming stands as the predominant mechanism behind the mass-transfer enhancement across the bubble surface during resonance. Single-frequency acoustic driving can momentarily amplify the rate of mass transfer by as much as two orders of magnitude; the overall mass transfer enhancement increases monotonically with the acoustic pressure amplitude and eventually plateaus. Frequency sweeps continuously looped in time prove a superior method of intensification. Provided that the sweep period is not too short, the direction of sweep matters: up-sweeps generally favour dissolution over growth, whereas down-sweeps favour growth over dissolution. An optimal sweep period that maximises the growth or dissolution process is shown to exist.