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

While analytical theories exist for predicting the negative radiation force on a sphere illuminated by an acoustic Bessel beam, addressing non-spherical objects and analyzing the influence of geometric features on the reversal of axial radiation force remains a challenge. Here, we introduce a theoretical framework combining addition theorem, conformal mapping, and partial-wave expansion to evaluate the acoustic radiation force on axisymmetric objects illuminated by Bessel beams. Our findings reveal that geometric asymmetry significantly influences axial radiation forces on sound-hard Mie objects in air. Negative radiation forces can emerge at half-cone angles exceeding approximately π/3 when objects possess a larger backward area. This threshold can shift to angles below π/3 with an increased backward area, whereas the negative forces nearly disappear when the forward area becomes dominant. Theoretical results align well with 3D numerical simulations. The statistical analysis further reveals that both existence percentage and overall magnitude of negative islands exhibit a significantly linear correlation with the area ratio of backward to forward regions. Finally, acoustic negative stiffness is observed in the beam-object system in pure carbon dioxide. This negative stiffness becomes more pronounced for objects with larger backward areas.