Soft Porous Silicone Rubbers as Key Elements for the Realization of Acoustic Metamaterials

Abstract

In this work, macroporous materials made of polydimethylsiloxane, a soft silicone rubber, are prepared using UV polymerization with an emulsion-templating procedure. The porosity of the final materials can be precisely controlled by adjusting the volume of the dispersed phase. We show that the porous structure of the materials is the template of the droplets of the initial emulsions. Mechanical tests show that the materials Young's moduli decrease with the porosity of the materials. Acoustic measurements indicate that, in such a porous elastomeric matrix, the sound speed also decreases dramatically as soon as the porosity increases to attain values of as low as 80 m/s. The results are compared to earlier ones on silica aerogels and are interpreted within the framework of a simple theoretical approach. We show that the very low sound speed value is a consequence of the low value of the polymer shear modulus. This explains why such porous soft silicone rubbers are so efficient at playing the role of slow-soft resonators in acoustic metamaterials. Moreover, the fast rate of polymerization of such UV-curable fluid allows for a facile shaping of the final material as beads or rods in microfluidic devices.1.

Soft porous silicone rubbers with ultra-low sound speeds in acoustic metamaterials

“…The Kuster-Toksöz model only providing quasi-static expressions for the effective properties of the porous media, it is not expected to give full account of dynamical (frequency-dependent) quantities such as attenuation coefficients. However, the fitting parameters found here ( α L  = 8.10 4   f  1.4 and α T  = 2.10 5   f   1.5 for ϕ  = 35%) are very close to previous ones measured in other soft porous silicone rubbers prepared using UV polymerization with an emulsion-templating procedure1218.…”

“…The dispersion effects are therefore negligible in this frequency range and then the use of the described cut-off procedure is justified here. At last, it is worth noting that the thermally-cured elastomer considered here exhibits phase velocities two times lower than that of previous soft porous silicone rubber materials polymerized by UV ( c L  ~ 80 m/s and c T  ~ 40 m/s)1218.…”

“…1, we also plotted the theoretical predictions issued from the long-wavelength approximation of the Waterman-Truell model as recently done18. It is worth noting that such a much more sophisticated model, based on multiple scattering theory, provides the same results as those obtained with the Kuster-Toksöz model based on single scattering theory.…”

“…The soft porous silicone rubbers used in this work were synthesized via an emulsion-templated procedure1824. The silicone network was formed by a polyaddition reaction of SiH and SiVi in a mixture with a platinum-based catalyst.…”

Negative-index metamaterials: is double negativity a real issue for dissipative media?

“…The first-ever reported 3D ultrasonic NIM [3] was a metafluid composed of micro-beads made of an ''ultra-slow'' soft porous silicone rubber [22] randomly dispersed in a host matrix. Its acoustic index n 0 has been shown to be negative over broad frequency ranges, depending on the micro-bead volume fraction U.…”