05 Mai – Thesis defense - Ludovic Alhaïtz

14 h Amphi F - Building A29 | University of Bordeaux (Talence campus)

Spiraling waves, confined phase singularities and acoustic spin density emerging during the scattering of an evanescent plane wave by an isolated resonant scatterer.

This work deals with the physical analysis of the scattering of an evanescent acoustic plane wave by an isolated and resonant spherical scatterer suspended in a fluid medium.
The scattering of an incident plane wave by an object of a size comparable to its wavelegth and with a circular cross-section generates circumferential waves that propagate along the surface of this object. In the case of an incident evanescent plane wave, symmetry breaking effects occur in the field. As a consequence, circular waves are preferentially excited compared to those which propagate in the opposite direction.
A circumferential wave field with angular propagation is then created inside the object. Due to the interferences of these waves, phase singularity lines appear where the field amplitude vanishes. Furthermore, the number of singularities is related to the order of the resonant scattering mode. In the external medium, the scattered pressure field has a spiral structure associated with omnidirectional energy transport.
A set-up has been developed to experimentally investigate the scattering of an evanescent plane wave by a resonant submillimetre-sized  droplet.
Evanescent plane waves are generated by the total internal refraction of a beam at the interface between two immiscible fluids. Then, thanks to the precision of the measurement device, the phase singularities present inside the object were detected and the field maps were successfully compared with theoretical predictions.
In this system, the energy flow circulates around the phase singularities and also presents a local rotation. These effects are related to unusual physical quantities contributing to the angular momentum of the field.
For the first time, an acoustic spin density is observed theoretically and experimentally inside a millimetre-sized droplet immersed in an evanescent field.
This quantity results from the elliptical displacement of medium particles undergoing an evanescent plane wave, and is proportional to the acoustic radiation torque that can be applied to sub-wavelength objects.
These results could have important implications for the non-contact translation or rotation of objects using evanescent fields, and could  be also exploited for acoustic imaging.

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