Shape distortion of an acoustically forced gas microbubble

The parametrically induced shape distortion of a micron sized gas bubble in water driven by a temporally sinusoidal pressure field in an axisymmetric geometry is considered using a model which accounts for nonlinear shape mode interactions. For a fixed driving frequency and considering initial bubble radii smaller than those which give rise to the prolate/oblate dominated shape via the fundamental resonance, shape distortion due either to synchronous or higher order harmonic resonances is identified. Considering cases where the parametric instability growth saturates, the resultant finite amplitude oscillations of the synchronously excited shape mode are found to be nearly sinusoidal, but shape modes excited via higher order harmonic resonances are found to consist of a number of frequency components. In the latter case, as the initial bubble radius is reduced, the order of the harmonic resonance causing the parametric excitement is found to increase, causing a shift in the constitute frequency components of the parametrically excited shape mode.