Confined phase singularities reveal the spin-to-orbital angular momentum conversion of sound waves

TL;DR

This paper demonstrates the conversion of spin to orbital angular momentum in sound waves through confined phase singularities at fluid interfaces, combining experimental and numerical evidence with implications for acoustic manipulation.

Contribution

It reveals a novel acoustic process of angular momentum conversion involving confined phase singularities, supported by experimental and numerical validation.

Findings

Confirmed spin-to-orbital angular momentum conversion in acoustic waves.

Identified confined phase singularities at fluid interfaces.

Demonstrated potential applications in particle manipulation and acoustic sensing.

Abstract

We identify an acoustic process in which the conversion of angular momentum between its spin and orbital form takes place. The interaction between an evanescent wave propagating at the interface of two immiscible fluids and an isolated droplet is considered. The elliptical motion of the fluid supporting the incident wave is associated with a simple state of spin angular momentum, a quantity recently introduced for acoustic waves in the literature. We experimentally observe that this field predominantly forces a directional wave transport circling the droplet's interior, revealing the existence of confined phase singularities. The circulation of the phase, around a singular point, is characteristic of angular momentum in its orbital form, thereby demonstrating the conversion mechanism. The numerical and experimental observations presented in this work have implications for the fundamental understanding of the angular momentum of acoustic waves, and for applications such as particle manipulation with radiation forces or torques, acoustic sensing and imaging.