Sonophysical mechanisms of bubble fragmentation near an ultrasonically vibrating boundary: influence of frequency and amplitude on interfacial intensification.

Ultrasonic cavitation has been increasingly employed to intensify wet dust removal by enhancing gas-liquid interfacial renewal and local mixing. However, the coupled effects of ultrasonic frequency and vibration amplitude on bubble collapse dynamics, fragmentation behavior, and cavitation sustainability remain insufficiently quantified. In this study, a numerical investigation is conducted to elucidate the collapse and breakup characteristics of a millimeter-scale single bubble near an ultrasonically vibrating wall, representing a typical cavitation-assisted scrubbing environment. A two-phase Volume of Fluid (VOF) model is used to resolve transient interface deformation, asymmetric collapse, and microjet formation under ultrasonic excitation. The effects of ultrasonic frequency (20-40 kHz) and vibration amplitude (5-30 μm) on bubble morphology, air volume fraction evolution, flow-field velocity, and fragmentation degree are systematically examined. The results show that collapse is dominated by directional contraction toward the vibrating boundary and the formation of high-speed penetrating microjets, which govern bubble breakup and interfacial generation. Increasing frequency and amplitude intensifies fragmentation and jet impact strength; however, excessive excitation promotes rapid depletion of residual bubble structures and suppresses sustained cavitation activity. Within the investigated range, the combination of 25 kHz and 20 μm provides a favorable balance between collapse intensity and cavitation persistence, enabling sustained fragmentation and enhanced interfacial complexity. Our study offers mechanistic insights and parameter-selection guidance for ultrasonic cavitation-assisted intensification of wet dust scrubbing and related gas-liquid-solid processes.