Scattering of acoustic evanescent waves by spherical resonant objects and induced radiation force

Abstract

Evanescent waves are particular solutions of the propagation equation. These waves are heterogenous plane waves which propagate along a given direction (generally the direction of an interface) with a higher spatial frequency content than homogenous waves. According to the dispersion relation, they have an exponential decay of amplitude in an orthogonal direction. A well known application in optics or acoustics is the near field microscopy which consists in detecting evanescent components of the field to improve the image resolution. Such waves can be generated in a fluid or solid medium from a beam oriented beyond the critical angle of total internal reflection, by Bragg scattering through a periodic interface, or by modes conversions in waveguides. Here we consider the scattering of an incident evanescent field by a fluid or elastic sphere in order to expose the benefits of exploiting their special characteristics for innovative applications. To analyse the scattering of an incident evanescent plane wave, the field is decomposed in the spherical functions basis. Compared to the case of a homogeneous plane wave, additional scattered modes are excited and have a significant contribution in the resulting total field. For a given particle radius, Mie resonances occur at the same frequencies than for homogeneous plane waves, but with an altered amplitude in the wavenumber space that can be tuned by increasing the degree of evanescence of the incident wave. Due to the exponential decay of amplitude in the incident field, the scattered radiation was observed to present exotic structures, especially a rotating phase around the scatterer. This results could have important implications for the contactless manipulation of objects with acoustic radiation forces and torques, or for the remote detection of buried obstacles in the naval sector.