Negative radiation force on sound-hard non-spherical objects: Enhanced by geometric asymmetry and formation of negative stiffness system.

This article presents a theoretical framework to evaluate the acoustic radiation force exerted by Bessel beams on non-spherical, axisymmetric objects, emphasizing the role of geometric asymmetry in reversing axial radiation forces. Using a combination of partial-wave expansion, conformal mapping, and the translation addition theorem, the study quantifies the influence of the area ratio between backward and forward regions (Ar) on the emergence of negative radiation forces, finding that objects with larger backward areas (Ar > 1) can experience such forces, while symmetric objects rarely do. The model incorporates attenuation effects in media like pure carbon dioxide, revealing that negative radiation forces increase as objects approach the beam source, resulting in a negative stiffness system tunable by beam pressure amplitude. The theoretical predictions align closely with full 3D numerical simulations, suggesting potential applications in contactless manipulation of non-spherical objects and noise control technologies.