The compression-only behavior of coated microbubbles in a wall restricted flow.

Abstract

The impact that the onset of the compression-only behavior of lipid shelled contrast agents bears on their dynamic interaction with a rigid wall under acoustic disturbances is investigated numerically in the context of axisymmetry. Wall presence is seen to not significantly affect the onset of compression-only since it only reduces the time frame required to trigger the effect. The standoff distance from the wall bears no significant effect on the amplitude threshold except that as it is reduced, it favors asymmetry by altering the compressed buckled shape around which the bubble oscillates. Above the amplitude threshold for parametric shape mode excitation, the onset of compression-only in the vicinity of a rigid wall typically interrupts the process of entrapment by reversing the direction of motion via the positive pressure drug that is generated as a result of the emerging concave upwards buckled shapes. Below this amplitude threshold, symmetric shapes or asymmetric shapes that are concave downwards continue to translate towards the wall where they perform saturated trapped pulsations around nearly spherical flattened or concave downwards buckled shapes. The latter shapes perform compression-only type pulsations and arise on the longer time scale required for the destabilization of the nearly spherical initially trapped shapes. Phase diagrams are constructed identifying regions of trapped pulsations, compression-only response, and microbubble collapse, in the parameter space defined by sound amplitude and shell viscoelastic properties.