Coupled microbubble oscillation induced selective particle manipulation

Acoustic tweezers utilize acoustic radiation force or acoustic streaming to enable versatile, non-invasive particle manipulation, offering non-contact and label-free operation. Oscillating microbubbles serve as secondary acoustic sources to generate secondary acoustic radiation force and microstreaming for microparticle manipulation, and this approach has received increasing attention. While extensive research has focused on single-bubble dynamics, the collective behavior of acoustically coupled microbubbles remains elusive. Here, we developed the coupled microbubble oscillations (CMOs) device to achieve selective particle manipulation and revealed the underlying physical mechanisms of CMOs. By arranging a hexagonal microbubble configuration, a virtual acoustofluidic potential well was established at the hexagon center. Moreover, unlike traditional potential wells induced by acoustic pressure gradients, this virtual acoustofluidic potential well is dominated by microstreaming-induced drag forces, which are three orders of magnitude larger than those of acoustic radiation forces. By exploiting the size-dependent scaling of these competing forces, we achieved the separation of 1-μm and 10-μm microparticles within the virtual potential well, with larger particles preferentially trapped at the center and smaller particles selectively organized in the peripheral regions. This work not only reveals the interaction among microbubble oscillation but also demonstrates a novel principle and methodology for advanced particle sorting and assembly, advancing the next-generation microfluidic devices.