Interaction of a two-microbubble system with a rough boundary under ultrasonic excitation

Investigating the dynamic behaviour of microbubbles under ultrasonic excitation is valuable in the biomedical and industrial fields, but the effect of boundary roughness on the interaction between microbubbles remains ambiguous. In this work, the coupled dynamics of a two-bubble system near a rough boundary are investigated, and a coupled vibration model with boundary roughness parameters, ultrasonic excitation parameters, and microbubble parameters is established. The dynamic responses of the double microbubble, the secondary Bjerknes force and the evolution of the stress/strain at the rough boundary at different roughness levels are analysed using the finite element method. The results indicate that localized flow field perturbations induced by rough boundaries alter the microbubble vibrational response and the stress/strain response at the boundary. The strength of the interaction between microbubbles and boundaries relies on the acoustic pressure amplitude, excitation frequency, boundary roughness, distance between two microbubbles, etc. A low frequency and large amplitude generate larger microjets, resulting in greater stress on the boundary. The asymmetric distribution of microbubbles is more prone to rupture and generates greater stress on the boundaries. The model also reveals that the roughness level has a significant nonlinear response to the boundary-layer stress field. Materials with greater surface roughness are more resistant to cavitation erosion, whereas those with lower surface roughness facilitate cavitation damage. The model improves the understanding of microbubble–boundary interactions in realistic environments.