  The dynamics of bubble clouds induced by high-intensity focused ultrasound
are investigated in a regime where the cloud size is similar to the ultrasound
wavelength. High-speed images show that the cloud is asymmetrical; the bubbles
nearest the source grow to a larger radius than the distal ones. Similar
structures of bubble clouds are observed in numerical simulations that mimic
the laboratory experiment. To elucidate the structure, a parametric study is
conducted for plane ultrasound waves with various amplitudes and diffuse clouds
with different initial void fractions. Based on an analysis of the kinetic
energy of liquid induced by bubble oscillations, a new scaling parameter is
introduced to characterize the dynamics. The new parameter generalizes the
cloud interaction parameter originally introduced by d'Agostino and Brennen
(1989). The dynamic interaction parameter controls the energy localization and
consequent anisoptropy of the cloud. Moreover, the amplitude of the far-field,
bubble-scattered acoustics is likewise correlated with the proposed parameter.
Findings of the present study not only shed light on the physics of cloud
cavitation, but may also be of use to quantification of the effects of
cavitation on outcomes of ultrasound therapies including HIFU-based
lithotripsy.
