The influence of compliant boundary proximity on the fundamental and subharmonic emissions from individual microbubbles.

The proximity of a solid-liquid boundary has been theoretically predicted to affect nonlinear microbubble emissions, but to date there has been no experimental validation of this effect. In this study, individual microbubbles (n = 15) were insonicated at f = 11 MHz as a function of offset distance from a compliant (agarose) planar boundary by employing an optical trapping apparatus. It was found that fundamental scattering increases while subharmonic scattering decreases as the microbubble approaches the boundary. Although a microbubble-boundary model can predict the qualitative trends observed for a subset of encapsulation properties, further modeling efforts are required to completely model compliant boundary-microbubble interactions.