Drying kinetics and acoustic properties of soft porous polymer materials
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Drying kinetics and acoustic properties of soft porous polymer materials R. Kumar1 · Y. Jin2,5 · S. Marre3 · O. Poncelet2 · T. Brunet2 · J. Leng4 · O. Mondain‑Monval1 Accepted: 16 September 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract We describe a method for the fabrication of acoustic metasurfaces, which is based on soft porous polymer materials. The materials are obtained using an emulsion templating technique, which allows for the fabrication of soft porous polymers with fully controlled porosity values between 0 and 30%. Our approach involves the polymerization of water-in-silicone emulsions with controlled water volume fractions. The obtained wet solid monolith samples are dried using three different methods. Due to the softness of the polymer matrix, and like in polyHIPE hydrogels or silica aerogels, the first method—regular air drying—leads to a collapse of the material and we present a complete experimental study of the observed kinetics as well as a model to account for the observed results. We show that this model can catch the kinetics characteristics. Then, using two alternative drying techniques, H2O2-assisted and supercritical drying, we show that it is possible to obtain materials with fully controlled porosities. The speed of sound—or equivalently the material acoustic index—inside the material being dependent on its porosity, we obtain a gradient-index acoustic material by spatially controlling the porosity distribution along the two dimensions of these metasurfaces. Their ability in terms of wavefront shaping is then demonstrated through a deflecting experiment performed in water with a sample having a thickness five times smaller than the incident acoustic wavelength at ultrasonic frequencies. Keywords Porous materials · Drying methods · Emulsion templating · Metasurfaces · Acoustic metamaterials
1 Introduction Metasurfaces are 2D materials of sub wavelength thickness that can provide non-trivial local phase shifts or amplitude modulation (or gradient) of the waves. Such devices enable a full spatial control of the wavefront. For example, a metasurface with appropriate properties can deflect or focus a * O. Mondain‑Monval [email protected] 1
University of Bordeaux, CNRS, UMR 5031 CRPP, 33600 Pessac, France
2
University of Bordeaux, CNRS, Bordeaux INP, ENSAM, UMR 5295 I2M, 33405 Talence, France
3
University of Bordeaux, CNRS, Bordeaux INP, UMR 5026 ICMCB, 33600 Pessac, France
4
University of Bordeaux, CNRS-Solvay, UMR 5258 LOF, 33600 Pessac, France
5
Present Address: School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai, P.R. China
beam. In other cases, it may also be used to produce twisted beams [1]. Initially introduced for electromagnetic waves, the concept has been extended to acoustics [2] and other areas of wave physics. Following those tracks, acoustic gradient index (GRIN) metasurfaces, i.e. surfaces in which the acoustic index varies as a function of position in the material, have been t
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