RESUMEN

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.

RESUMEN

Spherical silica xerogels are efficient acoustic Mie resonators. When these sub-wavelength inclusions are dispersed in a matrix, the final metafluid may display a negative acoustic refractive index upon a set of precise constraints concerning material properties, concentration, size, and dispersity of the inclusions. Because xerogels may sustain both pressure and shear waves, several bands with negative index can be tailored.

RESUMEN

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.

RESUMEN

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.

RESUMEN

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.

Asunto(s)

Fluorocarburos/química , Sonido , Ultrasonido , Emulsiones , Modelos Teóricos , Movimiento (Física) , Aceites , Tamaño de la Partícula , Presión , Dispersión de Radiación , Procesamiento de Señales Asistido por Computador , Análisis Espectral , Vibración

RESUMEN

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.