Optical Observations of Acoustical Radiation Force Effects on Individual Air Bubbles

Abstract—Previous studies dealing with contrast agent microbubbles have demonstrated that ultrasound (US) can significantly influence the movement of microbubbles. In this paper, we investigated the influence of the acoustic radiation force on individual air bubbles using high-speed photography. We emphasize the effects of the US param-eters (pulse length, acoustic pressure) on different bub-ble patterns and their consequences on the translational motion of the bubbles. A stream of uniform air bubbles with diameter ranging from 35 m to 79 m was gener-ated and insonified with a single US pulse emitted at a frequency of 130 kHz. The bubble sizes have been chosen to be above, below, and at resonance. The peak acoustic pressures used in these experiments ranged from 40 kPa to 120 kPa. The axial displacements of the bubbles produced by the action of the US pulse were optically recorded us-ing a high-speed camera at 1 kHz frame rate. The experi-mental results were compared to a simplified force balance theoretical model, including the action of the primary radi-ation force and the fluid drag force. Although the model is quite simple and does not take into account phenomena like bubble shape oscillations and added mass, the experimen-tal findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displace-ment varies with the size and the density of the air bub-bles, reaching a maximum at the resonance size of 48 m. The predicted displacement values differ by 15 % from the measured data, except for resonant bubbles for which the displacement was overestimated by about 40%. This study demonstrates that even a single US pulse produces radi-ation forces that are strong enough to affect the bubble position. I