Born approximation of trapping forces by acoustical Bessel and vortex fields.

Acoustic radiation forces have been used to trap various objects for fundamental studies and practical applications. Born approximation method, originally introduced to solve quantum scattering problems, is herein extended to analyze trapping forces exerted by two- and three-dimensional acoustic Bessel and vortex fields on spherical and nonspherical objects of arbitrary size. The results are compared with the conventional models like the partial wave expansion and Gorkov force potential. It is shown that for weakly scattering objects (such as common soft biological particles surrounded by fluids), the Born approximation can make predictions for the trapping forces on objects whose characteristic lengths are even up to multiple wavelengths of the sound beams. With the aid of the approximation, the Gorkov force potential is applied to analyze and gain insights into trapping forces on large objects far beyond the original Rayleigh scattering regime. The effects caused by the beam parameters, object shape, and orientation on the trapping behaviors are revealed. This work is useful for the further study of acoustic radiation forces and will guide the experiment of simplified acoustic tweezers on arbitrary-shaped particles.