Spiraling Waves, Acoustic Spin Density and Detection of Phase Singularities in Droplets Immersed in Evanescent Acoustic Fields
Résumé
We report experimental developments on the scattering of acoustic evanescent waves by an isolated particle. Compared to the scattering of homogeneous plane waves, additional oscillation modes are excited and particularly lead to the generation of spiraling scattered waves propagating angularly in the far field. Interestingly, phase singularities may appear inside the scattering particle as a consequence of interfering circumferential waves. The topological charge of these singular waves depends on the targeted scattered mode, excited by adjusting the driving frequency for a fixed particle size. We conducted experiments at ultrasonic frequencies on a resonant droplet immersed in a non-miscible host fluid to evidence these topological singularities. Evanescent waves were generated by total reflection at a two fluid interface at which the droplet was closely deposited. The field resulting from the scattering is shown to carry a transverse angular momentum of orbital nature which could bring an additionnal degree of freedom than classical vortex beams. Furthermore, the emergence of topological singularities in the field could be also linked to an acoustic spin density, important for applications involving radiation force and torque.