Soft 3D acoustic metamaterials

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One of the current challenges in the field of metamaterials is to extend beyond electromagnetism in other areas such as acoustics [1]. Soft matter techniques coupled with microfluidics provide a unique tool to take up this challenge because they allow for the production of 3D locally resonant (random) materials composed of various soft resonators [2]. The Mie (or cavity) resonators are interesting key elements for acoustic metamaterials since they may induce strong acoustic resonances provided large sound-speed contrasts between the inclusions and the host matrix [3]. For example, the use of “slow” (deformable) ferrofluid-droplets allows for the tuning of sharp Mie resonances, thanks to external magnetic fields [4]. For much higher sound-speed contrasts, we have recently shown that strong Mie resonances of “ultra-slow” beads made of soft porous silicone rubbers [5] could induce strong dispersion effects leading to materials with exotic (zero, negative) values of the acoustic index [6]. [1] Wegener, Science 342, 939–940 (2013). [2] Brunet et al., Science 342, 323–324 (2013). [3] Li and Chan, Phys. Rev. E 70, 055602 (2004). [4] Brunet et al., Phys. Rev. Letters 111, 264301 (2013). [5] Zimny et al., Langmuir 31, 3215–3221 (2015). [6] Brunet et al., Nature Mater. 14, 384–388 (2015).