Periodic onset of bubble shape instabilities and their influence on the spherical mode

Bubbles insonified by ultrasound can exhibit shape deformations and no more oscillate upon a purely spherical mode. Understanding and predicting the appearance of surface oscillations is essential for typical applications based on the acoustic response of bubbles (e.g. the use of contrast agents in echographic imaging), partly because shape modes can contribute to the monopole emission of sound by the bubble. In this work, we present a set of controlled experiments aiming at recording high-resolved temporal dynamics of surface oscillations of micrometer-sized bubbles. Single bubbles nucleated by a focused laser pulse and trapped in an amplitude-modulated ultrasonic field are captured by a high-speed CCD camera with an acquisition rate of 180 kfps. A proper expansion of the bubble interface coordinate over the Legendre's polynomials basis allows to obtain all the modal amplitudes corresponding to spherical and nonspherical oscillating modes. The slowly varying amplitude-modulated ultrasonic field causes beats of the spherical oscillations which alternate between low-amplitude phases and high-amplitude phases. Surface instabilities develop on the highamplitude phases while they disappear on the low-amplitude ones, thus leading to a periodic onset of the bubble shape modes. Analyzing the experimental results reveals that several shape modes exist simultaneously in the bubble dynamics, some of them not being parametrically triggered by the breathing mode. Comparisons with numerical simulations allow to support that these modes come from nonlinear interactions between spherical and nonspherical oscillations. Another consequence of nonlinear coupling effects observed in our experiments is the substantial modification of the breathing mode induced by the surface oscillations.