Acoustic radiation torque of a Bessel vortex wave on a viscoelastic spherical shell nearby an impedance boundary

Abstract

In this investigation, a comprehensive analytical formalism is presented for the acoustic radiation torque exerted on a viscoelastic spherical shell nearby an impedance boundary in a Bessel vortex wave field, whose incident direction is perpendicular to the boundary. Based on the sound scattering theory, an infinite series expansion of the axial acoustic radiation torque is derived by applying the series expansion method, the image theory and the translational addition theorem. Computations of the non-dimensional axial acoustic radiation torque for a viscoelastic polyethylene shell are considered with emphasis on the effects of the reflecting coefficient of the interface, the shell-boundary distance, the shell's relative thickness and the half-cone angle. It is shown that the high-amplitude radiation torque peaks exist resulting from different vibration modes at resonance scattering frequencies, which are significantly enhanced when the boundary becomes increasingly rigid. The periodical change of acoustic radiation torque is also observed as the shell moves along the wave axis. With the increase of wave order, the high-amplitude “islands” shift to larger half-cone values. The results of this study can improve our understanding of the acoustic radiation torque behaviors for viscoelastic particles near the boundary, which is commonly encountered in biomedical ultrasound and fluid dynamics.