Translational motions and radial oscillations of a polymer-coated microbubble in the focal cross-section of focused acoustic vortex

The polymer-coated microbubble stimulated by focused acoustic vortex has great potential in enhancing the targeted drug delivery, but the dynamic behaviors of the microbubble in the focal cross-section of focused acoustic vortex requires further investigation. A mathematical model, which accounts for the acoustic radiation force, changes in the instantaneous acoustic field and the effects of shell properties, is developed to investigate the translational motions and radial oscillations of a polymer-coated microbubble excited by a focused acoustic vortex. Results show that in the focal cross-section, the microbubble in all cases moves in a direction consistent with the change of acoustic phase. The microbubble located at a relatively small initial distance from the focus moves toward the focus. However, once the initial distance exceeds a certain threshold, the microbubble moves away from the focus. Furthermore, a larger topological charge of the focused acoustic vortex results in a larger initial distance threshold. In regard to radial oscillations, the oscillation amplitude of microbubble decreases with an increase in the driving cycle due to its translational motion. Moreover, increasing the initial external radius of the microbubble or the velocity amplitude of the n^th sectorial transducer enhances the translational motions and radial oscillations of the microbubble. Conversely, these dynamic behaviors are suppressed by increasing the topological charge, viscosity of the surrounding medium, shell viscoelasticity and thickness. This work comprehensively investigates the polymer-coated microbubble dynamics in the focal cross-section of focused acoustic vortex. It provides preliminary insights into the mechanism of combining focused acoustic vortex technology with polymer-coated microbubble for targeted drug delivery, which may contribute to optimization of experimental parameters settings.