Confined Phase Singularities Reveal the Spin-to-Orbital Angular Momentum Conversion of Sound Waves.

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 Letter 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.

Experimental demonstration of negative refraction with 3D locally resonant acoustic metafluids.

Negative refraction of acoustic waves is demonstrated through underwater experiments conducted at ultrasonic frequencies on a 3D locally resonant acoustic metafluid made of soft porous silicone-rubber micro-beads suspended in a yield-stress fluid. By measuring the refracted angle of the acoustic beam transmitted through this metafluid shaped as a prism, we determine the acoustic index to water according to Snell's law. These experimental data are then compared with an excellent agreement to calculations performed in the framework of Multiple Scattering Theory showing that the emergence of negative refraction depends on the volume fraction [Formula: see text] of the resonant micro-beads. For diluted metafluid ([Formula: see text]), only positive refraction occurs whereas negative refraction is demonstrated over a broad frequency band with concentrated metafluid ([Formula: see text]).

Elaboration of soft porous ultrasound insulators.

A simple and easy way is proposed for the fabrication of a highly attenuating composite material for underwater acoustics. The approach involves the introduction of porous polymer beads into a polyurethane matrix. The porous beads are prepared through an emulsion-templating approach, and two different processes are used. The first one uses microfluidics to synthesize beads of controlled diameter and porosity. The control over the bead size allows the selection of the frequency range where the material exhibits the highest acoustic attenuation. The second one uses a double emulsion approach and allows for the production of much larger quantities of beads. Both approaches yield materials exhibiting much higher acoustic absorption than the one obtained using the most commonly used micro-balloon inclusion. We present both the synthesis procedures and the structural and acoustic characterizations of the beads and the final acoustic materials.

Soft porous silicone rubbers with ultra-low sound speeds in acoustic metamaterials.

Soft porous silicone rubbers are demonstrated to exhibit extremely low sound speeds of tens of m/s for these dense materials, even for low porosities of the order of a few percent. Our ultrasonic experiments show a sudden drop of the longitudinal sound speed with the porosity, while the transverse sound speed remains constant. For such porous elastomeric materials, we propose simple analytical expressions for these two sound speeds, derived in the framework of Kuster and Toksöz, revealing an excellent agreement between the theoretical predictions and the experimental results for both longitudinal and shear waves. Acoustic attenuation measurements also complete the characterization of these soft porous materials.

Soft porous silicone rubbers as key elements for the realization of acoustic metamaterials.

In this work, macroporous materials made of polydimethylsiloxane, a soft silicone rubber, are prepared using UV polymerization with an emulsion-templating procedure. The porosity of the final materials can be precisely controlled by adjusting the volume of the dispersed phase. We show that the porous structure of the materials is the template of the droplets of the initial emulsions. Mechanical tests show that the materials Young's moduli decrease with the porosity of the materials. Acoustic measurements indicate that, in such a porous elastomeric matrix, the sound speed also decreases dramatically as soon as the porosity increases to attain values of as low as 80 m/s. The results are compared to earlier ones on silica aerogels and are interpreted within the framework of a simple theoretical approach. We show that the very low sound speed value is a consequence of the low value of the polymer shear modulus. This explains why such porous soft silicone rubbers are so efficient at playing the role of slow-soft resonators in acoustic metamaterials. Moreover, the fast rate of polymerization of such UV-curable fluid allows for a facile shaping of the final material as beads or rods in microfluidic devices.1.

Soft 3D acoustic metamaterial with negative index.

Many efforts have been devoted to the design and achievement of negative-refractive-index metamaterials since the 2000s. One of the challenges at present is to extend that field beyond electromagnetism by realizing three-dimensional (3D) media with negative acoustic indices. We report a new class of locally resonant ultrasonic metafluids consisting of a concentrated suspension of macroporous microbeads engineered using soft-matter techniques. The propagation of Gaussian pulses within these random distributions of 'ultra-slow' Mie resonators is investigated through in situ ultrasonic experiments. The real part of the acoustic index is shown to be negative (up to almost - 1) over broad frequency bandwidths, depending on the volume fraction of the microbeads as predicted by multiple-scattering calculations. These soft 3D acoustic metamaterials open the way for key applications such as sub-wavelength imaging and transformation acoustics, which require the production of acoustic devices with negative or zero-valued indices.

Design of a fluorinated magneto-responsive material with tuneable ultrasound scattering properties.

In this work, we describe the preparation of emulsions of fluorinated ferrofluid droplets suspended in a yield-stress hydrogel (Bingham fluid) with potential applications for ultrasound (US) spectroscopy and imaging. Fluorinated ferrofluids were obtained using an original multi-step process leading to an appropriate suspension of magnetic nanoparticles (MNPs) coated by a layer of fluoroalkylsilane in fluorinated oil. The efficiency of the sol-gel coating reaction was assessed by several methods including infrared and X-ray photoelectron spectroscopy, small angle neutron scattering and magnetometry. The resulting suspension of silanized-MNPs behaves as a true fluorinated ferrofluid, remaining stable (i.e. a monophasic suspension of well dispersed MNPs) in magnetic inductions as high as 7 T. These ferrofluids were employed to prepare monodisperse emulsions in a Bingham gel using a robotic injection device. Using ultrasound spectroscopy, we show that the emulsion droplets behave as Mie-type acoustic wave resonators due to the high sound-speed contrast between the droplets and the matrix. When subjected to a magnetic field, the ferrofluid droplets elongate in the field direction, which in return modifies the acoustic response of the material. The resonance frequency peaks scale as the inverse of the emulsion droplet size encountered by the wave propagation vector. These results might open a new road towards the realisation of ultrasound contrast agents guided by magnetic fields and with a tuneable attenuation spectrum.

Impact of polydispersity on multipolar resonant scattering in emulsions.

The influence of size polydispersity on the resonant acoustic properties of dilute emulsions, made of fluorinated-oil droplets, is quantitatively investigated. Ultrasound attenuation and dispersion measurements on various samples with controlled size polydispersities, ranging from 1% to 13%, are found to be in excellent agreement with predictions based on the independent scattering approximation. By relating the particle-size distribution of the synthesized emulsions to the quality factor of the predicted multipolar resonances, the number of observable acoustic resonances is shown to be imposed by the sample polydispersity. These results are briefly discussed into the context of metamaterials for which scattering resonances are central to their effective properties.

Using dispersion equation for orthotropic media to model antiplane coherent wave propagation in cracked solids.

Attention is focused on the propagation of antiplane coherent wave obliquely incident on mutually parallel and randomly distributed cracks. A fundamental question in this study concerns the ability of describing the coherent wave propagation in all directions from the knowledge of the effective material properties along the effective principal directions, only. Its relevance is illustrated by considering two cases of coherent wave propagation: homogeneous and inhomogeneous waves. For both cases, the effective phase slownesses approximated from the dispersion equation specific for orthotropic homogeneous media are compared to reference results obtained from a direct calculation considering waves obliquely incident on cracks. This work reveals that the effective stiffnesses of this dispersion equation have to be dependent on the propagation direction of the incident wave in order to make this equation consistent.

Tuning Mie scattering resonances in soft materials with magnetic fields.

An original approach is proposed here to reversibly tune Mie scattering resonances occurring in random media by means of external low induction magnetic fields. This approach is valid for both electromagnetic and acoustic waves. The experimental demonstration is supported by ultrasound experiments performed on emulsions made of fluorinated ferrofluid spherical droplets dispersed in a Bingham fluid. We show that the electromagnet-induced change of droplet shape into prolate spheroids, with a moderate aspect ratio of 2.5, drastically affects the effective properties of the disordered medium. Its effective acoustic attenuation coefficient is shown to vary by a factor of 5, by controlling both the flux density and orientation of the applied magnetic field.

Further results for antiplane scattering by a thin strip.

A result, which does not appear to be available elsewhere, concerning the far-field scattering of antiplane waves by a single thin crack in an elastic solid is obtained in this letter. Specifically, the angular shape function is derived, with scattering coefficients expressed in terms of the crack-opening displacement. With this function, numerical values for effective speed and attenuation through a distribution of parallel cracks are obtained.

Effective material properties for shear-horizontal acoustic waves in fiber composites.

The effective dynamic properties of composites made of elastic cylindrical fibers randomly distributed in another elastic solid can be evaluated with plane shear-horizontal acoustic waves. In this paper, it is shown that the effective mass density and the effective shear stiffness are complex valued and frequency dependent. Simple formulas are derived for these effective quantities. The low-frequency limit of these formulas is found to be in agreement with physical expectations. The derivation is based on the multiple-scattering approach of Waterman and Truell, where each cylinder of finite cross section is replaced with an equivalent line scatterer. Numerical results are presented for the effective mass density and effective shear stiffness for various values of frequency, cylinder concentration, and elastic properties of the cylinders and matrix.