Mechanical and Civil Engineering Seminar

Ph.D. Thesis Defense

Abstract: 

Wave propagation in periodic structures has been studied for centuries; for example, Newton derived the velocity of sound based on a linear lattice. Recently, advanced manufacturing techniques have led to the fabrication of geometrically complex architected materials with acoustic properties unattainable by their constituent materials. Such rationally designed structures are often called acoustic metamaterials and they can be engineered to transmit, block, amplify, or redirect acoustic waves. Subwavelength building blocks, typically periodic (but not necessarily so), can be assembled into effectively continuous materials to manipulate dispersive properties of vibrational waves in ways that differ substantially in conventional media. This thesis investigates rationally designed acoustic metamaterials, periodic in 1D or 3D, and how acoustic wave propagation can be controlled by these artificially structured composite materials.

We first explore DNA-inspired 1D periodic helical mechanical metamaterials and study mode hybridization of the material with perturbation. We identify three distinctive normal modes when the helical metamaterials are excited with dynamic loading. We demonstrate the ability to vary the helical metamaterials' dispersion properties by controlling the geometrical structure and mass distribution. By locally adding eccentric and denser elements in the unit cells, we perturb the moment of inertia of the system and introduce centro-asymmetry. This allows us to control the degree of mode coupling and the width of subwavelength band gaps in the dispersion relation, which are the product of enhanced local resonance hybridization.

We then study 3D periodic metamaterials with microscale architectures, such as microlattices, which can be designed for tailorable acoustic properties and extreme quasi-static mechanical response. When coupled with pressure waves in surrounding fluid, the dynamic behavior of microlattices in the long wavelength limit can be explained in the context of Biot's theory of poroelasticity. We exploit elastoacoustic wave propagation within 3D-printed polymeric microlattices to design a gradient refractive index lens for underwater wave focusing. A modified Luneburg lens index profile adapted for ultrasonic wave lensing is demonstrated via the finite element method and underwater testing, showcasing a computationally efficient poroelasticity-based design approach that enables accelerated design of acoustic wave manipulation devices.

Lastly, we show that microlattice metamaterials can be successfully implemented as conformal skull replacements for brain imaging. Functional ultrasound imaging enables sensitive, high-resolution imaging of neural activity in freely behaving animals and human patients. However, the skull acts as an aberrating and absorbing layer for sound waves, leading to most functional ultrasound experiments being conducted after skull removal. A microscale 2-photon polymerization technique is adopted to fabricate a conformal acoustic window with a high stiffness-to-density ratio and sonotransparency. Long-term biocompatibility and lasting signal sensitivity are demonstrated over a long period of time (> 4 months) by conducting ultrasound imaging in mouse models implanted with the metamaterial skull prosthesis.