An efficient semi-analytical procedure to calculate acoustic radiation force and torque for axisymmetric irregular bodies

Abstract

<p>With the attributes of being biocompatible, label-free, and contact-free, sound has been widely used to manipulate micro-objects, such as cells, soft matters, and living things, by the acoustic radiation force and torque arising from wave scattering. The analytical theory exists for the acoustic radiation force and torque on separable geometries, including sphere and ellipsoid. However, it is still a challenge to handle irregular surfaces subject to non-orthogonal and inseparable boundary conditions. Here, we present a calculation method for an axisymmetric geometry with irregular cross-section excited by a time-harmonic plane wave with arbitrary incidence in the inviscid fluid. The method is based on conformal mapping from the irregular surface to a sphere in the new coordinate system. In this way, the separation of variables can be used to solve the corresponding Helmholtz wave equation subjecting to the spherical boundary conditions in the new coordinate system. The radiation force and the torque are asymptotically obtained using the far-field, closed-form solutions. The method is validated by comparisons with full 3D numerical solutions over a wide range of frequencies and incident angles. With a typical discrepancy of less than 5%, the proposed method is much more efficient than the full numerical simulations (via finite element method). Furthermore, it is found that the radiation force acting on different geometries follows the same tendency. The difference is that the presence of the radiation torque ensures that the symmetry axis of the irregular bodies is coincident with or orthogonal to the wave propagating direction.</p>