Role of gas nuclei in ultrasonic atomization in acoustic fountains: mechanisms and experimental evidence

Ultrasonic atomization, particularly in the form of acoustic fountains, is widely used in various fields, but its underlying mechanisms remain incompletely understood, with ongoing debates between wave-driven and bubble-driven hypotheses. This study introduces classic cavitation nucleus theory into high-frequency ultrasonic atomization research, focusing on the "gas-nucleus-cavitation-atomization" chain to provide reproducible evidence for the bubble-driven hypothesis. Experimental setups with a 1.7 MHz ultrasonic transducer, capillary tubes, and high-speed cameras were used to manipulate gas nuclei in acoustic fountains through pre-embedding, injection, and pouring methods. Results demonstrate a strong spatiotemporal correlation between gas nuclei and atomization-localized atomization occurs precisely at gas nucleus locations, and increasing gas nuclei content enhances atomization intensity. Simulated vibration sources (broken capillary tips) reproduce atomization characteristics similar to natural ultrasonic fountain atomization, confirming the critical role of vibrating bubbles derived from gas nuclei. Additionally, surface protrusions of acoustic fountains act as micro-resonators, and acoustic focusing within these structures promotes gas nucleus growth into vibrating bubbles, triggering atomization. Inhibiting surface fluctuations suppresses atomization, further supporting the mechanism. This study clarifies the pivotal role of gas nuclei in ultrasonic fountain atomization, providing a theoretical basis for precise control and efficient application of ultrasonic atomization technology.