Soft Acoustic Metamaterials: from 3D locally resonant metafluids to soft porous gradient index metasurfaces

Soft acoustic metamaterials are classes of functional materials for acoustics achieved by means of soft matter techniques such as microfluidics, chemical formulation or self-assembly [1]. In this talk, I will review our achievements in that field that allowed us to fabricate the first 3D acoustic metamaterial with a negative index [2]. To do that, we took benefit from the strong low-frequency Mie-type resonances of “ultra-slow” micro-beads randomly dispersed in a water-based gel. The soft porous silicone rubber material used to make these soft porous particles, not only exhibits ultra-low sound speeds ( 40m/s), but also shows a strong dependence of its sound speed on the porosity [3], providing thus a material with an acoustic index varying over a wide range of values. By assembling thin stripes made of these soft porous silicone rubbers with different porosities, we have recently proposed to fabricate soft gradient-index metasurfaces, alternatively to our 3D locally resonant metafluids, for wavefront shaping. The ability of these flat and ultra-thin acoustic lenses will be demonstrated through various underwater ultrasound experiments such as beam steering, ultrasound focusing and vortex beam generation [4]. Finally, I will report on 3D underwater ultrasound focusing experiments performed with a quasi-flat high-index acoustic lens [5] whose focal length can be tuned by hardening or softening the material thanks to UV light [6].