Drying kinetics and acoustic properties of soft porous polymer materials

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.