Surface modes with controlled axisymmetry triggered by bubble coalescence in a high-amplitude acoustic field

Abstract

When two bubbles encounter each other in a moderate ultrasound field they coalesce into a single bubble that shows purely spherical oscillations. For a sufficiently large acoustic field, this coalescence can lead to sustained nonspherical oscillations of the resulting bubble. We experimentally capture the time-resolved dynamics of the coalesced bubble, starting with the moment of film rupture. This allows the transient and steady-state regimes of the oscillating bubble to be studied. The amplitude dynamics for both of these regimes are successfully compared to numerical modeling, taking into account the coupling of volume and surface modes. Initial conditions for each surface mode are taken from the Legendre polynomial projection of the experimentally obtained bubble shape immediately after film rupture. We also observe that the symmetry axis of the zonal spherical harmonics is defined by the coalescence. The axis is identical to the rectilinear translational motion of the two approaching bubbles before impact. This high-amplitude coalescence technique provides a unique opportunity to study axisymmetric and sustained nonspherical bubble oscillations under controlled initial conditions.