Soft acoustic metamaterials are classes of functional materials for acoustics that are achieved by means of various soft-matter techniques such as chemical formulation & microfluidics [1]. In this talk, I will review our recent advances in that field that allowed us, notably, to report the first soft 3D acoustic metamaterial with a negative acoustic index [2]. This metafluid was composed of “ultra-slow” porous micro-beads, randomly dispersed in a water-based gel, acting like strong low-frequency Mie-type resonators. The soft porous silicone rubber material used to make these resonant particles, not only has ultra-low sound speeds ( 40m/s), but also exhibits a strong dependence of the longitudinal speed of sound on the porosity. Actually, the acoustic index of these soft porous materials can vary from 1.5 up to 40 when the porosity increases from 0 to 35% [3]. By assembling thin stripes made of these porous elastomers with different porosities, we have created a new class of flat acoustic lenses, namely soft gradient-index metasurfaces, designed for ultrasonic wavefront shaping. Finally, I will report some recent underwater experiments demonstrating beam steering, beam focusing, as well as vortex beam generation in free space [4].

• [1] T. Brunet et al., Science 342, 323 (2013)
• [2] T. Brunet et al., Nature Materials 14, 384 (2015)
• [3] A. Ba et al., Scientific Reports 7, 40106 (2017)
• [4] Y. Jin et al., Nature Communications 10, 143 (2019)