Design, fabrication, and characterization of acoustic metamaterials with embedded resonators
Acoustic metamaterials provide the remarkable ability to control, direct, and manipulate sound waves. Within this infant field, a promising approach to fabricate locally resonant acoustic metamaterials is the use of resonators composed of a heavy core surrounded by a rubber shell dispersed in an epoxy matrix. At their resonant frequency, the cores vibrate 180° out-of-phase with the matrix, and a band gap in transmission is observed making these materials excellent sound absorbers. A significant challenge in the study and adoption of these materials is the lack of techniques to easily fabricate resonators with a wide range of sizes and properties. Here, we present a robust yet simple technique to fabricate resonators with diameters ranging from 50 µm to 5 mm from core-shell drops generated in microfluidic and millifluidic devices. We started by fabricating resonators with core diameters ranging from 50 µm to 1 mm from double emulsion drops composed of a concentrated ceramic suspension in the core (inner drop) surrounded by a UV-crosslinkable rubber shell (outer drop) using microcapillary microfluidic devices. The double emulsion drops were collected and exposed to UV to crosslink the shell material forming resonators with resonant frequencies ranging from 100 kHz to 25 kHz based on the core mass. Even lower resonant frequencies were obtained by fabricating resonators with core diameters ranging from 1.2 mm to 1.7 mm from core-shell drops extruded in air from a coaxial nozzle at rates up to 6 drops/minute. The effects of core density were studied by utilizing suspensions composed of ceramic particles of increasing density including silica, alumina, and lead zirconate titanate (PZT). The transmission properties of the acoustic metamaterials made with resonators with different core diameters, core materials, and level of ordering within the matrix, were measured using a piezoelectric transducer setup capable of testing from 1 kHz to 50 kHz. For example, acoustic metamaterials composed of randomly dispersed 1.4 mm alumina-core resonators at about 25 vol% concentration in epoxy showed a well-defined band-gap around 11 kHz. A finite element model was also developed by collaborators to capture the acoustic transmission physics of these materials. Overall this technique offers a robust path for the fabrication of acoustic resonators and locally resonant acoustic metamaterials.
Youngblood, Purdue University.
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