An air bubble driven by ultrasound can become shape-unstable through a parametric instability. We report time-resolved optical observations of shape oscillations (mode n=2 to 6) of micron-sized single air bubbles. The observed mode number n was found to be linearly related to the ambient radius of the bubble. Above the critical driving pressure threshold for shape oscillations, which is minimal at the resonance of the volumetric radial mode, the observed mode number n is independent of the forcing pressure amplitude. The microbubble shape oscillations were also analyzed numerically by introducing a small nonspherical linear perturbation to a Rayleigh-Plesset-type equation, capturing the experimental observations in detail.

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 parameters (pulse length, acoustic pressure) on different bubble 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 generated 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 using a high-speed camera at 1 kHz frame rate. The experimental results were compared to a simplified force balance theoretical model, including the action of the primary radiation 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 experimental findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displacement varies with the size and the density of the air bubbles, 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 radiation forces that are strong enough to affect the bubble position.

Abstract-We have investigated surface vibrations generated by ultrasound excitation of individual unencapsulated micron-sized bubbles. In addition, we present surface modes (n = 2 and 3) observed for phospholipid-coated ultrasound contrast agents excited through excitation of radial modes at frequencies between 1 and 4 MHz. Even higher modes of vibration (up to mode 5) are observed for coated microbubbles at insonation frequencies of 10 and 19 MHz. The potential relevance of surface modes for medical ultrasound is discussed, including the possible implications for current theoretical models of ultrasound contrast agents.

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