By: Alen Pavlič and Thierry Baasch Acoustic forces occurring in resonant microfluidic chambers allow the handling of micrometer-sized particles. Trapping of nanoparticles has been shown for high particle concentrations and after preseeding the chamber with larger particles. We show that the trapping mechanism is due to synergy between hydrodynamic shielding and acoustic forces. Acoustic interactions are significant only when seed particles are involved and are negligible when