Reversal of mid-air acoustic radiation forces: From low- to high-pressure trapping

Acoustic levitation in mid-air has conventionally relied on confining particles in low-pressure regions, such as standing wave nodes or twin traps. However, analyses of horizontal acoustic radiation forces at larger distances from the sound source reveal a reversal phenomenon wherein particles are instead attracted to high-pressure regions within a plane parallel to the transducer array. Through numerical simulations, we show that this effect appears not only in focused or Bessel beams but also when the phase signatures are applied. Although numerical calculations suggest that focused beams could enable mid-air levitation, stable experimental realizations have proven challenging. Here, we demonstrate for the first time that a single-sided configuration using a Bessel beam can stably levitate and translate particles in high-pressure regions, even at distances exceeding those achievable by twin traps. We further investigate whether our Bessel beam approach aligns with numerical optimal conditions for maximizing trapping stiffness. These findings underscore the potential of harnessing high-pressure regions to enhance acoustic manipulation in air, opening new avenues for single-sided levitation strategies.