Millifluidic Synthesis of Core–Shell Resonators For Tunable Sonic BandGaps in Acoustic Metamaterials

Millimeter‐scale ceramic–core/silicone–shell resonators were fabricated by gravity‐assisted coaxial dripping and embedded in epoxy bars to form locally resonant acoustic metamaterials. The process produced core–shell drops at approximately 5–6 drops/min, with usable yields greater than 95% and diameter coefficients of variation below 10%. Alumina () and lead zirconate titanate (PZT) suspensions were used as the core materials, while a UV‐crosslinkable silicone rubber formed the shell. Although the PZT suspension had a higher density than the alumina suspension, the smaller PZT cores resulted in core masses similar to those of the alumina resonators; consequently, both resonator types produced attenuation near 11 kHz. Piezoelectric transmission measurements on epoxy bar samples showed attenuation bands near the finite‐element‐predicted resonant frequencies of 11.8 kHz for alumina–core resonators and 10.3 kHz for PZT‐core resonators. Increasing the alumina–core resonator loading from 13.0 to 52.7 vol% broadened the attenuation band and increased the attenuation magnitude. These results demonstrate that gravity‐assisted coaxial dripping provides a high‐yield route for producing millimeter‐scale core–shell resonators that can be integrated into polymer composites for sonic‐frequency acoustic attenuation.