We investigated the occurrence of nonspherical oscillations (or surface modes) of coated microbubbles, used as ultrasound contrast agents in medical imaging, using ultra-high speed optical imaging. We used optical tweezers to trap the bubbles far from any boundary, enabling a controlled study of the nonspherical oscillations of free-floating bubbles. Nonspherical oscillations appear as a parametric instability and display subharmonic behavior: they oscillate at half the forcing frequency, which was fixed at 1.7 MHz in this study. Surface modes are shown to preferentially develop for a bubble radius near the resonance of radial oscillations. In the studied range of acoustic pressures, the growth of surface modes saturates at a level far below bubble breakage. With the definition of a single, dimensionless deformation parameter, the amplitude of nonspherical deformation is quantified as a function of the bubble radius and of the acoustic pressure.

B. Dollet, S. M. van der Meer, V. Garbin, N. de Jong, D. Lohse, M. Versluis, Ultrasound in Medicine and Biology 34, 1465 (2008)