Non-spherical oscillations and induced microstreaming of single bubble in standing wave field.

The transition of a freely suspended microbubble from spherical to nonspherical oscillation in a standing-wave acoustic field, together with the transient flow generated during this transition, was investigated using a combined experimental and numerical approach. In the experimental component, a single bubble was levitated near the center of the acoustic field using an ultrasonic levitation system. High-speed imaging captured the shape transition of the bubble from spherical to nonspherical oscillation and the trajectories of surrounding particles induced by the nonspherical bubble oscillation. These trajectories were subsequently superimposed to reconstruct the acoustic microstreaming structures. In the numerical component, simulations based on the Navier-Stokes equations and arbitrary Lagrangian-Eulerian (ALE) method with moving grids were performed to reproduce the transition from spherical to nonspherical oscillation and to resolve the transient vortex flow evolving throughout this transition. Furthermore, parametric studies revealed that driving frequency, bubble radius, and driving pressure significantly modulate the nonspherical oscillation modes and the resulting transient vortex topology. The mechanisms underlying vortex structure formation and the distribution characteristics of shear stress during multimode-coupled bubble oscillations were elucidated. These findings provide valuable insights for the application of acoustic microstreaming in particle manipulation, targeted drug delivery, and related fields.