Acoustic metafluids with multiple negative-index bands

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 [1]. 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 [2]. 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 [3]. 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.

[1] Brunet et al., Science 342, 323–324 (2013). [2] Brunet et al., Nat. Mater. 14, 384–388 (2015). [3] Raffy et al., Adv. Mater. DOI: 10.1002/adma.201503524 (2016).