Acoustic Metamaterials at the Microscale

R. Sun


Micro-architected materials allow for tunability of extreme static mechanical prop- erties such as stiffness, Poisson’s ratio, or strength. However, dynamic and acoustic properties of micro-architected materials remain largely unexplored, partially because it is challenging to measure their response at these scales. Dispersion resulting from Bragg scattering occurs at wavelengths which are dictated by the characteristic dimen- sions of the metamaterials, while local resonance remains wavelength-independent. Therefore, micro-architected materials have the potential to allow control of mechan- ical waves both at high (MHz) and medium-range (kHz) frequencies. Here, we design, fabricate, and characterize micro-architected materials with tun- able mechanical and acoustic properties in the megahertz regime. Using a two-photon lithography prototyping method, we explore the response of a class of architected ma- terial morphologies with varied mass distribution, features down to ∼ 1.5 µm, and unit cell sizes of 15 µm. We demonstrate that decoupling mass and stiffness by strate- gically placing micro-inertia affects the effective stiffness scaling of this class of acous- tic metamaterials at the microscale. We present novel measurement techniques for wave velocity of three-dimensional architected materials that employ laser-ultrasonic principles, demonstrating a tunable range of wave velocities around 1000 m/s for dif- ferent designs in a wide range of relative densities. We then validate their acoustic response numerically with Bloch wave analysis to determine their dispersion relation and rod-wave velocities. Our results provide a baseline to map the tunable acoustic metamaterial design space at the microscale and megahertz regime. These materi- als could have important implications in acoustic devices in microelectromechanical systems, biomedical imaging, and microscale waveguides.