Observations of the effects of radiation forces on individual air bubbles with high-speed photography

Background: Previous studies dealing with contrast agent microbubbles have demonstrated that an ultrasound (US) burst can significantly influence the flow of microbubbles and therefore the Doppler frequency shifts. This study presents the effect of the radiation force on individual air bubbles with various diameters using high-speed photography. Method: A stream of uniform air bubbles with a diameter ranging from 31 μm to 80 μm was insonified with a single US pulse. The transmitted pulse contained 10 cycles at 130 kHz. The measured acoustic pressures ranged from 68 kPa up to 180 kPa. The maximal axial displacements of the bubbles produced by the action of the US pulse were optically recorded using a high-speed camera at a frame rate of 1 kHz. The camera was synchronized with the US burst. The experimental results were compared to a force balance theoretical model including the action of the primary radiation force and the fluid drag force. Results: Experimental findings agree with the theoretical predictions: the axial displacement varies with bubble size, reaching a maximum at the resonance size of 48μm. For an acoustic pressure of 100 kPa the maximal displacement was 83 μm for resonant bubbles. For bubble sizes of 37, 55, 69 and 75 μm the observed displacements were 33, 49, 19 and 10 μm respectively. The measured displacement increases linearly with the burst length and nonlinearly with the transmitted acoustic pressure. The calculated values from the model differ by 15 % from the measured ones except for resonant bubbles where the displacement was overestimated up to three times. Conclusions: This study demonstrates that even a single US pulse produces significant radiation forces on the bubbles that are strong enough to affect their position. Moreover, they appear to be highly dependent on the size and the concentration of the bubbles.