The role of the numerical aperture to enhance acoustic trapping in levitation with focused standing waves

The numerical aperture (NA) is a key parameter in optical tweezers, but its role in acoustic trapping has been largely overlooked. Here, we investigate experimentally and theoretically the impact of NA on the performance of airborne standing-wave acoustic levitators formed by transducer arrays mounted on opposing spherical caps. Acoustic radiation forces are computed for representative geometries, and trapping performance is characterized by the axial and lateral stiffness at equilibrium. We show that levitators with the same NA exhibit very similar trapping behavior, even when their curvature, number of transducers, and overall dimensions differ. In addition, particle-size effects are analyzed in terms of NA, demonstrating that Mie particles shift their equilibrium positions as NA varies. These results identify NA as an important optimization parameter for the design of acoustic levitators.