The extraordinary properties of acoustic (random) metamaterials, such as negative refractive index, originate from low frequency resonances of sub-wavelength particles. While most of these functional materials are fabricated by mechanical engineering, we have recently shown that soft matter techniques coupled with microfluidics open a new synthesis route for acoustic metamaterials especially for ultrasonics . As a demonstration, we have achieved 3D-bulk acoustic metafluids with an alternatively low, zero, and negative index by producing large amounts of calibrated soft porous micro-spheres, acting like strong Mie resonators . The wide variety of physico-chemical processes offered by chemical engineering allows for the tuning of the resonant particle properties over a broad range of mechanical/acoustical parameters. In this talk we show that, according to a fine control of the Poisson coefficient of the macro-porous resonators, it is not only possible to achieve soft acoustic metamaterial with one negative band, but also with two separate and tunable ones . As both their width and depth depend on particle properties, the “soft approach” should benefit the design and fabrication of building-block composites with specific extreme properties required in some targeted applications, such as spatial control of sound for beamforming or cloaking.
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