Investigation of cavitation cloud characteristics near a bubble in a high-intensity focused ultrasound field

High-intensity focused ultrasound (HIFU) is a representative application of ultrasound cavitation, owing to its spatial specificity and versatility. When HIFU targets a region, ultrasonic cavitation and associated mechanical stress cause material damage. The cavitation cloud dynamics are strongly affected by nearby boundaries, yet the mechanisms of bubble shielding and HIFU focusing remain insufficiently understood. This study numerically investigates cloud cavitation dynamics in a focused ultrasound field containing a pre-set bubble. Results identify three developmental stages: initiation, driven by reflections from the pre-set bubble; reverse growth, influenced by backward scattered waves; and radial growth, governed by lateral scattering. HIFU focusing properties crucially affect all stages—initiation timing correlates with the focal effect and larger focusing angles accelerate radial expansion. Both the pre-set bubble and the cavitation cloud exhibit strong shielding, blocking over 90% of ultrasound energy, which is redistributed mainly as backward-scattered waves. The scattering and shielding effects depend on the cloud's equivalent radial position, increasing with its size. These insights into the development and scattering of cavitation clouds provide theoretical support for improved control of cavitation behavior.