Functional Acoustic Coatings with Soft Metamaterials – BRENNUS

The manufacturing techniques borrowed from the world of soft matter open up many avenues for the production of new materials, as they are numerous and varied in terms of the processes involved and the physico-chemical properties of the constituents (polymers for example). Fortified by our recent developments on macroporous resonators of the polymer foam type, real elementary bricks of our acoustic metamaterials, we can seriously consider the manufacture of several acoustic devices thanks to the wide range of acoustic index that we have reached experimentally.
This project is a multidisciplinary project between three Bordeaux laboratories involving experts in the physics of waves, soft matter and micro-fluidics. The consortium has been actively collaborating for 10 years around acoustic metamaterials obtained by soft matter techniques.

The «materials« challenge consists in developing a new class of acoustic coatings based on concepts such as metamaterials, which are easy to shape and produce in large quantities. The «meta-matter« consists of a two-phase polymer/water emulsion (therefore shapeless) which must be able to be dried efficiently in order to achieve target acoustic properties in a well-controlled manner of the final porous medium, then is shaping by different techniques depending on the application such as microfluidics (resonant beads for metamaterials), molding (gradient index materials for metasurfaces), soft lithography. In this project, the possibilities of synthesis and structuring of materials in the form of acoustic devices are very wide thanks to the many techniques that are used.
The «wave physics« challenge concerns the design and demonstration of targeted acoustic functions. The functions chosen, in connection with long-awaited technological breakthroughs in the field, will have as their primary vocation the spatial control of fields for spatial filtering, wavefront modulation.

The major scientific results of the project (reported in 7 scientific journal articles and 1 patent) are summarized by:
1) the mastery of two techniques for drying macroporous materials with an elastomer matrix allowing targeted and stable final acoustic properties (non-retraction of the porosities): a) by supercritical CO2 drying, b) by a technique involving hydrogen peroxide (H2O2);
2) demonstration of negative refraction by a locally resonant 3D metamaterial;
3) fabrication and ultrasonic measurements of metasurfaces with 3D functionality: flat lenses for 3D focusing; flat lenses for the generation of acoustic vortices;
4) manufacture of conventional (refractive) quasi-flat lenses with very short focal lengths.

BRENNUS research resulted in:
1) acceptance of a patent for France, and planned extension abroad on the theme of acoustic metasurfaces;
2) three new partnerships including the ANR ASTRID PANAMA project (2018-2021) with Naval Group on the theme of anechoic coatings for underwater acoustics.

The project has enabled a certain advance in the mastery of the manufacture of solid and soft materials with a wide range of acoustic index values. The different concepts involved in the macroscopic physical properties of these materials (high indices by controlled porosity, resonant scattering by microbeads, multidimensional gradient index, etc.) are all possibilities for creating devices or structures with targeted acoustic functionalities. If the direct applications in the ultrasonic field (Mega Hertz) require additional work so that the devices are more effective against the «parasitic« absorption effects, an application in the tens kilo Hertz range very quickly started with an ANR DGA ASTRID project specifically concerning anechoic coatings and masking in underwater acoustics (partnership between this consortium and Naval Group).

The main scientific results of this project have been highlighted in 7 peer-reviewed publications in international scientific journals. A slightly more «general public« communication was also carried out with professional players in the world of sound and engineering through 1 summary article in the series Techniques de l'Ingénieur, and 1 generalist article on metasurfaces in the review Acoustique et Techniques of the Centre d’Information sur le Bruit (CidB) and the French Society of Acoustics. In addition, the principle of graded index metasurfaces in the context of soft porous matter has been awarded a national patent, and an international one in the process of being acquired.

Acoustic Metamaterials constitute a new and promising generation of materials whose unconventional characteristics, based on "exotic" values of their acoustic index (from 0 to infinity, imaginary, even negative), open the way to many advanced applications such that wave-field spatial control (for high-resolution imaging and beamforming), ultra-absorption and cloaking (for insulation and stealth). Among all classes of metamaterials, the locally resonant metamaterials take advantage of low-frequency resonances of "small" inclusions. Thanks to those local resonators these metamaterials allow the full control of long-waves while keeping a small size (sometimes very small), giving rise to the name of "sub-wavelength" metamaterials.
At that time, there is a great deal of interest for applications with those acoustic materials especially for insulation in the audible domain and for stealth in underwater acoustics. Also, but with probably a lower societal and strategic pressure, the issues for ultrasonic instrumentation and imaging are potentially significant. Nevertheless acoustic metamaterials are still mostly at the stage of conceptual objects of laboratory.

The route of soft-matter techniques opens up many ways of designing and fabricating new materials thanks to their great variety of processes and to the physical/chemical properties of the involved constituents (polymers for instance). Turning to good account our recent successful developments of macro-porous resonators (polymeric foam micro-beads), which are a key-element for acoustic metamaterials, we are at the stage where we can envisage the making of many different types of metamaterial-based devices relying on a large range of index-values (now experimentally available).

This proposal is a strongly multi-disciplinary project between three labs in Bordeaux, expert in wave-physics, soft-matter and microfluidics. The consortium has more than 5 years of experience in joint-research on the topic of soft-metamaterials. This long collaboration coupled to the geographical proximity and the developed complementary skills is a key-point for challenging both the material and the acoustic aspects of the project.
The Material challenge is to develop a new class of acoustic coatings based on the metamaterials concept, which are easily processable and up-scalable. First the meta-matter can be a fluid-based dispersion like paint and turned afterwards into solid-but-soft coating upon polymerization over a large variety of surfaces. Second, the meta-coating can be structured by using soft-lithography and shaped by molding. In that project the possibilities for the resonators synthesis and their structuring into a metamaterial-device are quite vast.
The Wave-Physics challenge concerns the design and the experimental proof of targeted functions of several demonstrators. The chosen functions, in connection with further breakthrough technologies, deals with first: sub-wavelength absorption in the context of insulation and stealth; and second: wave-field spatial control for beamforming/front-shaping and cloaking.

The flow scheme of BRENNUS is the following:
1. Chemistry and synthesis of micro-resonators according to some criterions (size, shape, high calibration, surface treatment, mass production...) and formulation of the host matrix;
2. Structuring and shaping metamaterials using soft-matter techniques in order to achieve paint-like raw metamaterial, flexible coatings and molded structures;
3. Realization of three types of demonstrators:
a. Sub-wavelength meta-coating for sound insulation and stealth
b. Transducer caps for beamforming in ultrasonics
c. Anisotropic meta-layers for cloaking structures