Investigation of the nonlinear propagation of ultrasound through a bubbly medium including multiple scattering and bubble-bubble interaction: Theory and experiment

Understanding of the propagation of ultrasound through a bubbly medium is a challenging task because of the nonlinear dynamics of the bubbles and their effect on the attenuation and sound speed of the medium. The majority of the studies on this subject apply linear models, which will generate inaccurate results, especially at higher-pressure excitations. These studies have also ignored the effect of bubble-bubble interaction and nonlinear multiple scattering. In this work, we have numerically simulated the attenuation and sound speed of a bubbly medium by solving our recently developed nonlinear model. An efficient method to investigate the nonlinear bubble-bubble interaction and multiple scattering is developed, and this phenomenon is included the numerical investigations through considering a cluster of 130 randomly distributed interacting bubbles with sizes derived from experimental measurements. Broadband experimental attenuation measurements of monodisperse lipid-coated microbubble solutions were performed with peak acoustic pressures ranging within 10-100kPa. The bubble solutions had mean diameters of 4-6 micron and peak concentrations of 1000 to 15000 bubbles/ml. At lower concentrations (with minimal bubble-bubble interactions), predictions of the model (attenuation and sound speed vs frequency) in the absence of interaction are in good agreement with experimental measurements. At higher concentrations, secondary peaks in the attenuation and sound speed diagrams as a function of frequency appear. Through considering the bubble-bubble interactions, the numerical results can predict the quantitative and qualitative changes in the attenuation and frequency as well as the generation of secondary peaks.