Formation and Stabilization of Bulk Nanobubbles Triggered by Ultrasonic Irradiation under Different Conditions

The generation and stabilization of nanobubbles (NBs) are crucial concerns, considering their great potential for applications in various fields. Nonetheless, research on the stabilization of bulk nanobubbles (BNBs) generation across various systems under ultrasonic irradiation is relatively few. For example, how dissolved gases and different conditions affect the evolution of BNBs in the acoustic field remains unclear. Therefore, this study focused on generating and stabilizing BNBs over time under various conditions including ultrasonic frequency, power, and dissolved gases in both open and closed systems. First, for a given solution, the concentration of BNBs would increase with higher ultrasonic power and lower ultrasonic frequency. Furthermore, a considerably elevated concentration of BNBs was obtained in closed systems relative to open systems, which may be attributed to a closed system providing a more stable environment for nucleation growth, thus facilitating the generation and stabilization of BNBs. More surprisingly, by changing dissolved gas saturation, we found that in gas-saturated water, the concentration of BNBs becomes higher than in the other two saturations: supersaturated and undersaturated water. A detailed study also found that the concentration of formed BNBs differs based on the positions of vessel, and more BNBs will be formed at the bottom or upper side of the vessel, indicating bubbles easily nucleate near the vessel wall and at the gas-liquid interface. This study provides essential insights into the principles of the generation and stabilization of NBs under ultrasonic fields, potentially expanding application ranges and improving the efficiency of ultrasonic irradiation.