Manmade metamaterials based on holes in the ground, or solid columns buried inside it, may be, to some extent, ideal metamaterials. As such their properties should be very close to those obtained through analytical or numerical approaches. Yet forests, collections of buildings in a city, and more generally any existing geophysical object approximated as a metamaterial, will deviate from ideal metamaterials due to two main factors:
the scatterers or resonators may be randomly positioned and,
their characteristic size may include a random distribution of resonant frequencies (height of the trees, materials of the buildings, etc.).
Within META-FORET, we wish to study at the scale laboratory the transition from ordered to disordered metamaterials, in the context of elastic waves propagating in solids, and more generally in terms of any wave interacting with resonant unit cells.
It is well known that the properties of the bandgap created by locally resonant metamaterials is insensitive to positional disorder, that is, to a random distribution of position of the unit cells. Yet this is true solely if the metamaterial is governed by interferences only. If there exist strong interactions between the unit cells of a metamaterial, it is possible to completely loose the properties of the latter when a large positional disorder is included. For example, we have shown that the bandgap created by elastic rods resonators coupled to asymmetric Lamb waves in a plate give raise to wide band gaps whether the sample are ordered or disordered (Rupin et al, 2014). This means that the physics underlying the metamaterial properties within META-FORET is based on interference effects only to first order.
Figure 1.
Soda can metamaterial under experimental investigation in a homemade anechoic room. Twelve speakers, connected to a multichannel soundcard, can insonify the metamaterial sequentially while a microphone mounted on a 2D translation stage records the wave field at any position in the near field of the sample
In WP2, we propose then to study at the lab scale the order-disorder transition in acoustic metamaterials with simple unit cells: soda cans. Part of this work will be studied within the PhD of Simon Yves, who started to work at Institut Langevin this year, while he will later on benefit from the support from the postdoc planned within the META-FORET project.
We will first focus our research on the addition of spatial disorder to locally resonant sonic crystals, in order to study the formation of localized modes near the band edges of these regularly-ordered media. To do so, low density soda cans samples will be studied in an anechoic chamber using arrays of speakers, microphones, and a 2D translation stage (see Fig. 1). We will perform frequency/space cartography of the acoustic wave field supported by a random medium constituted of Helmholtz (...)
In a second step, we will move from ordered to disordered metamaterials. In these types of composite media, it was shown that a local modification can be realized without altering the properties of the rest of the metamaterial, at the scale of the unit cell. Such local modifications can be realized by inserting unit cells whose resonant frequencies lie in the bandgap of the otherwise unmodified metamaterial. This allows to realize cavities, waveguides, or filters that can confine and (...)
Benefitting from the laboratory set-up and equipment facility at ISTerre, small-scale experiments will be performed with vertical rods attached to the plate in the configuration described in section "State of the art" Fig. 1. To complement experimental results at Institut Langevin, we will focus on cloaking and lensing effects with rods of different lengths and a spatial configuration of rods determined by numerical simulations (see WP3.1).
In practice, we ambition to perform numerous (...)
