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).
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.
Bianisotropy is common in electromagnetism whenever a cross-coupling between electric and magnetic responses exists. However, the analogous concept for elastic waves in solids, termed as Willis ...coupling, is more challenging to observe. It requires coupling between stress and velocity or momentum and strain fields, which is difficult to induce in non-negligible levels, even when using metamaterial structures. Here, we report the experimental realization of a Willis metamaterial for flexural waves. Based on a cantilever bending resonance, we demonstrate asymmetric reflection amplitudes and phases due to Willis coupling. We also show that, by introducing loss in the metamaterial, the asymmetric amplitudes can be controlled and can be used to approach an exceptional point of the non-Hermitian system, at which unidirectional zero reflection occurs. The present work extends conventional propagation theory in plates and beams to include Willis coupling and provides new avenues to tailor flexural waves using artificial structures.
We investigate the process of self-assembly, and the resultant structures in composites of silica particles with a hexagonal mesophase of a nonionic surfactant and water. We report a systematic ...transition in behavior when the particle size is increased relative to the characteristic mesophase spacing. Water dispersible cage-like silsesquioxanes that are molecular analogues of silica particles and are smaller than the mesophase spacing swell the space between the surfactant cylinders. Silica particles comparable to the characteristic hexagonal spacing partition into the hexagonal phase and into strandlike particulate aggregates. Even larger particles phase separate from the hexagonal phase to form particulate strands that organize with a mesh size comparable to the wavelength of visible light. This self-assembly is reversible and the particles disperse by breaking up the aggregates on heating the composite into the isotropic phase. On cooling from the isotropic phase into the hexagonal, the particles are expelled from the growing hexagonal domains and finally impinge to form strandlike aggregates. Unusually, the isotropization temperature is increased in the composites as the particles nucleate the formation of the hexagonal phase.
Soft Acoustic Metamaterials Brunet, Thomas; Leng, Jacques; Mondain-Monval, Olivier
Science (American Association for the Advancement of Science),
10/2013, Letnik:
342, Številka:
6156
Journal Article
Recenzirano
Soft materials that embed small resonators in a host material can dampen or focus sound.
Resonance phenomena occur with all types of vibrations or waves and may play a part in spectacular events, ...such as the collapse of structures—for example, the fall of the Broughton suspension bridge near Manchester in 1831 (
1
). Indeed, the oscillations of a structure submitted to harmonic excitation reaches its maximum amplitude at the resonance frequency ω
0
of the system. At low driving frequencies (ω < ω
0
), its response is in phase with the forcing but becomes out of phase just beyond (ω
0
< ω). Such an out-of-phase response has been exploited with “locally resonant materials” (
2
). The proposed strategy is to embed a large enough collection of identical mechanical resonators in a passive structure to control wave propagation. These features are used to reach unusual macroscopic behaviors such as ultradamping of noise or negative refraction for imaging (
3
).
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.
Polypeptide-based diblock copolymers forming either well-defined self-assembled micelles or vesicles after direct dissolution in water or in dichloromethane have been studied combining light and ...neutron scattering with electron microscopy experiments. The size of these structures could be reversibly manipulated as a function of environmental changes such as pH and ionic strength in water. Compared to other pH-responsive self-assembled systems based on “classical” polyelectrolytes, these polypeptide-based nanostructures present the ability to give a response in highly salted media as the chain conformational ordering can be controlled. This makes these micelles and vesicles suitable for biological applications: they provide significant advantages in the control of the structure and function of supramolecular self-assemblies.
PDF
