Directional scattering reinforced by acoustic bianisotropy and related acousto-mechanical effects*

First order bi-anisotropic effects in linear acoustics are connected with a coupling mechanism between monopole and dipole modes. This phenomena is known as Willis coupling and appeared in literature in 1985 [1]. We note that only recently it had started to receive attention. Also various acousto-mechanical engineering devices were highlighted in the literature, demonstrating precise control of subwavelength particles [2].

In this work we present theory based on the coupling mode theory and symmetry analyses digging deeper in the acoustic bianisotropy. The acoustic force in the first order approximation with radiation pressure correction was recently reinvestigated from the perspective of optical forces theoretical tools [3]. Particularly, the recoil force is proportional to the product of monopole and dipole modes, and, hence, is aligned with the induced dipole mode. By introducing, generally, bianisotropy in the system we are able to achieve steering force by just adjusting the phase between induced monopole and dipole. This fine tuning is possible by varying the properties of the subwavelength particle.

[1] Willis. Intern. Jour. of Solids and Structures 21.7 (1985)
[2] Hirayama et al. Nature 575.7782, 320-323 (2019)
[3] Toftul et al. PRL 123.18 (2019)