Nonlinear dynamics of a cavitation bubble pair near a rigid boundary in a standing ultrasonic wave field.

• A modified BEM model considering the viscous effect is used to study the micrometre-sized bubble pair in ultrasonic waves. • Nonlinear bubble deformation results in the shift of the bubbles' first order frequency. • The excitation frequency which is lower than the natural frequency induces the stronger collapse of the bubbles. • The vertical distribution of the bubble pair may result in better surface cleaning effect. The dynamics of a micrometer-sized bubble pair in water near a rigid boundary under standing ultrasonic wave excitation is investigated in this study. The viscous effect in the boundary layer at the air-water interface is considered following the viscous correction model. The evolution of the bubble surface at the collapsing stage of the bubble pair is presented for different parameter sets. The field pressure near the rigid boundary, which is induced by the oscillating bubble pair, and the high-speed water jet at the collapse stage, form the main focus of the analysis. This reveals that a horizontal configuration of the bubble pair retards the strength of the bubble jet towards the boundary, whilst a vertical configuration, especially with differently-sized bubbles, can enhance the bubble collapse. This study may help to understand the interaction of multiple bubbles in an acoustic field and its application to surface cleaning. [ABSTRACT FROM AUTHOR]