Transferred energy and acoustic spin density in fluid spherical core-shell scattering scenarios

Abstract

Sound wave scattering, along with radiation force and torque, are constantly in vogue topics for several distinct applications, such as small object manipulations and Acoustofluidics. Specially for scattering scenarios, far-field approximations are often considered for dealing with the sound waves far beyond the scattering objects. However, a more in-depth investigation of near and internal fields during a scattering process may be able to unravel dynamical and mechanical properties of the scattering objects. In this study, we propose a time-averaged study of the transferred energy and acoustic spin density in a fluid spherical core-shell. We derive the expressions for the internal waves in terms of spherical harmonics and obtaining Mie-like coefficients. Using those expressions, we analytically compute the time-averaged acoustic energy transferred by a plane wave. Furthermore, we use very recent results with regards to acoustic spin to bring a discussion about spin density transfer, properly computing the quantity for this scenario. This is a follow-up study for understanding acoustical properties of compressible fluids with different geometries, searching for electromagnetic parallels, such as Fano resonance regimes, that may be applicable for building or characterizing acoustic metamaterials.