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.

Tailoring of the porous structure of soft emulsion-templated polymer materials.

This paper discusses the formation of soft porous materials obtained by the polymerization of inverse water-in-silicone (polydimethylsiloxane, PDMS) emulsions. We show that the initial state of the emulsion has a strong impact on the porous structure and properties of the final material. We show that using a surfactant with different solubilities in the emulsion continuous phase (PDMS), it is possible to tune the interaction between emulsion droplets, which leads to materials with either interconnected or isolated pores. These two systems present completely different behavior upon drying, which results in macroporous air-filled materials in the interconnected case and in a collapsed material with low porosity in the second case. Finally, we compare the mechanical and acoustical properties of these two types of bulk polymer monoliths. We also describe the formation of micrometric polymer particles (beads) in these two cases. We show that materials with an interconnected macroporous structure have low mechanical moduli and low sound speed, and are suitable for acoustic applications. The mechanical and acoustical properties of the materials with a collapsed porous structure are similar to those of non-porous silicone, which makes them acoustically inactive.

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.

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.

TiO2 thin films self-assembled with a partly fluorinated surfactant template.

New TiO(2) films have been self-assembled on solid substrate by dip-coating using TiCl(4) as the titanium source and the partly fluorinated surfactant F(CF(2))(8)C(2)H(4)(OC(2)H(4))(9)OH as the liquid crystal template. By control over the dip-withdrawal speed, film thicknesses from a minimum of 43 nm were produced with rms roughnesses of 0.5-0.7 nm. The films were characterized by X-ray reflectivity, grazing incidence small-angle X-ray scattering, atomic force microscopy, contact angle measurements, and Raman spectroscopy. Their GI-SAXS patterns are characteristic of a 2-D hexagonal structure in which tubular rods of the fluorinated surfactant are packed hexagonally and aligned parallel to the substrate. Reflectivity and contact angle measurements of the as-prepared film indicate that a low-density hydrophilic TiO(2) surface presents to the air.

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.