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Dynamics of aperiodic elastic lattices

Aperiodic structures are organized, deterministic systems that are not periodic. These systems present attractive architectures for material design due to their compelling localized dynamics and structural properties which differ from their fully periodic or amorphous counterparts. Our group investigates these features by designing non-Euclidean and non-integer dimensional structures, such as hyperbolic and fractal lattices. Through theoretical and experimental investigations, we uncover novel passive localized wave phenomena owing to the unique architectural arrangements of these structures. Notable features include frequency-dependent localized wave propagation in hyperbolic lattices, which are preserved in the conformal mapping of these structures to beam-like domains. In fractal lattices, fractal spectral properties are observed with predictable spectral changes from lattice impurities which lead to fractal mode localization in space. This work ushers in a novel class of elastic metamaterials with highly localized states suitable for advanced wave control and vibration isolation.

Wave propagation through curved domains

This project advances our understanding of wave propagation in curved, thin elastic structures to enable the design of a new class of acoustic lenses and waveguides. Dynamic curvature reconfiguration is explored to induce a diversity of desired wave properties such as guiding, collimation, focusing and steering. Previously, gradient index materials produced by spatial variations of physical properties have received significant attention in optics and acoustics. Exploiting curved geometries broadens the design space while enabling advantages for reconfigurable acoustic devices otherwise unachievable. The key idea is that acoustic waves on thin shells tend to follow surface geodesics - curves on a surface that are locally the shortest paths between points - and that these geodesics diverge and converge according to the surface's intrinsic curvature. Our overarching objective is achieved via three aims that tightly integrate theory, design, fabrication, and characterization: 1) Understanding geometric lensing of acoustic waves on curved surfaces. 2) Designing acoustic waveguides for geometric lensing. 3) Reconfiguring shells to enable adaptive wave propagation.

Cranial Lamb waves for skull and brain diagnostics and therapy

Due to its morphological nature, the human skull exhibits a waveguide-like behavior that supports a Lamb wave motion similar to that observed in multilayered orthotropic plates. While the skull presents a barrier to pressure waves used in focused ultrasound (FUS) based treatments, which are usually limited to central regions of the brain, waves that exploit the quasi-bidimensional structure of the cranial bone can possibly create new treatment options for neurological conditions, especially at the brain periphery. In addition, they provide a non-invasive diagnostic tool for the skull that can overcome typical drawbacks of FUS such as local bone heating. Our research group theoretically and experimentally investigates the nature and characteristics of cranial Lamb waves with the aim to exploit their inherent dispersive features for various diagnostic and therapeutic applications. These include the mechanical characterization of the cranial bone and sutures, and the implementation of new transducer setups for enhanced intracranial ultrasound delivery which leverage Lamb mode conversion within the skull to achieve better spatial resolution and target small regions of the brain. 

Dynamics of quasiperiodic and quasicrystalline elastic metamaterials

Metamaterials are man-made artificial materials based on particular arrangements and combinations of known materials that produce unique properties not encountered in it’s individual constituents. While the vast majority of metamaterials are based on periodic designs based on a repeating unit cell, our group is currently investigating quasiperiodic and quasicrystalline metamaterials that break the periodicity paradigm. The first category is based on modulations of the properties of inclusions, such as point masses, springs, resonators or stiffeners, that are given by a deterministic non-periodic pattern. In the second case, quasicrystalline materials may exhibit symmetries which are not possible to obtain in periodic materials, such as 5,7 ,8 and 10-fold rotational symmetries. The investigation of the dynamics of such materials reveals intriguing properties such as topological bandgaps and localized vibration modes that can be manipulated by a few key parameters, and wave directionalities enabled by higher order rotational symmetries that expand the behavior known to be possible in periodic metamaterials.

Fundamental mechanisms of distributed bleed flow control over an aeroelastic wing

Flow-controlled, variable aerodynamic load distributions effected in flight by interactions between wing surfaces and the embedding flow that are regulated by surface-integrated distributed active bleed can enable a new class of adaptive, lightweight, agile, highly-deformable wings. The use of bleed actuation for aeroelastic control is new, and represents a significant departure from earlier approaches that have relied on direct mechanical deformation of the lifting surfaces or on moving control surfaces.  Our current efforts focus on the fundamental mechanisms of the coupled fluid-structure interactions between the bleed actuation and the flow over static and dynamic flexible wings. Additionally, applications of distributed bleed flow control are examined when applied to vibration control.