Nicholas Boechler

Nonreciprocal interactions in active matter are known to generate exotic mechanical behaviors such as odd elasticity and odd viscosity. However, these phenomena have largely been studied in isolation, raising a fundamental question: Is there a single system that embodies these distinct regimes of odd matter and can transition between phases, establishing a unified phase diagram for nonreciprocal active matter? To address this, we introduce a tunable robotic active matter platform, the Magnetomechanically Augmented Spinning roBotic (MASBot) collective, in which particle-level control of chirality, activity, and pairwise interactions enables access to distinct phases of odd matter. By continuously increasing repulsive forces relative to attractive and transverse forces, we experimentally map a transition from an odd elastic crystal to an odd viscous liquid, and then to a chiral active gas. We find that this latter phase forms a non-space-filling, nonreciprocal active gas stabilized by long-range hydrodynamic attractive forces, whose statistical signatures are consistent with those of a two-dimensional self-gravitating point vortex gas. Within these phases, adjusting spinning frequency and introducing spatially patterned activity allows us to fine-tune odd mechanical responses and tailor power spectra. Further polar and rotational symmetry breaking at the particle scale leads to novel emergent states such as phase separation and collective translation. Together, our system provides a fundamental experimental testbed for nonequilibrium physics and establishes a blueprint for treating robotic swarms as programmable states of matter, enabling functions that range from resilient structures to adaptive swarm reconfiguration.

Nonreciprocal interactions in active matter are known to generate exotic mechanical behaviors such as odd elasticity and odd viscosity. However, these phenomena have largely been studied in isolation, raising a fundamental question: Is there a single system that embodies these distinct regimes of odd matter and can transition between phases, establishing a unified phase diagram for nonreciprocal active matter? To address this, we introduce a tunable robotic active matter platform, the Magnetomechanically Augmented Spinning roBotic (MASBot) collective, in which particle-level control of chirality, activity, and pairwise interactions enables access to distinct phases of odd matter. By continuously increasing repulsive forces relative to attractive and transverse forces, we experimentally map a transition from an odd elastic crystal to an odd viscous liquid, and then to a chiral active gas. We find that this latter phase forms a non-space-filling, nonreciprocal active gas stabilized by long-range hydrodynamic attractive forces, whose statistical signatures are consistent with those of a two-dimensional self-gravitating point vortex gas. Within these phases, adjusting spinning frequency and introducing spatially patterned activity allows us to fine-tune odd mechanical responses and tailor power spectra. Further polar and rotational symmetry breaking at the particle scale leads to novel emergent states such as phase separation and collective translation. Together, our system provides a fundamental experimental testbed for nonequilibrium physics and establishes a blueprint for treating robotic swarms as programmable states of matter, enabling functions that range from resilient structures to adaptive swarm reconfiguration.

Topological metamaterials promise unprecedented wave control. Here, we theoretically and numerically investigate a one-dimensional Su-Schrieffer-Heeger (SSH) inspired stiffness dimer modified with a local resonator, which imparts a frequency-dependent effective stiffness to the unit cell. We demonstrate a two-step mechanism to create a topological local-resonance-induced band gap (LRG): first, a conventional Bragg-type band gap (BrG) is made topologically non-trivial via band inversion at a Dirac point; second, by tuning a dimerization parameter, the character of this non-trivial BrG is switched to that of an LRG via an intermediate flat band state. This process preserves the non-trivial topology without requiring gap closure within the LRG. Crucially, we find that when the resulting topological edge state intersects a characteristic frequency of the LRG -- specifically, an attenuation singularity where the effective stiffness vanishes -- it achieves extreme localization of vibrational energy. This state is confined to a single particle at the boundary, resulting in an inverse participation ratio of exactly unity, the theoretical limit for localization in a discrete system. Further, we demonstrate that while random disorder scatters the frequency of this mode, introducing tuned boundaries stabilizes the single-particle mode over a broad parameter range. Our findings provide a clear pathway to designing ultra-localized, topologically protected states in low-frequency regimes.

Topological metamaterials promise unprecedented wave control. Here, we theoretically and numerically investigate a one-dimensional Su-Schrieffer-Heeger (SSH) inspired stiffness dimer modified with a local resonator, which imparts a frequency-dependent effective stiffness to the unit cell. We demonstrate a two-step mechanism to create a topological local-resonance-induced band gap (LRG): first, a conventional Bragg-type band gap (BrG) is made topologically non-trivial via band inversion at a Dirac point; second, by tuning a dimerization parameter, the character of this non-trivial BrG is switched to that of an LRG via an intermediate flat band state. This process preserves the non-trivial topology without requiring gap closure within the LRG. Crucially, we find that when the resulting topological edge state intersects a characteristic frequency of the LRG -- specifically, an attenuation singularity where the effective stiffness vanishes -- it achieves extreme localization of vibrational energy. This state is confined to a single particle at the boundary, resulting in an inverse participation ratio of exactly unity, the theoretical limit for localization in a discrete system. Further, we demonstrate that while random disorder scatters the frequency of this mode, introducing tuned boundaries stabilizes the single-particle mode over a broad parameter range. Our findings provide a clear pathway to designing ultra-localized, topologically protected states in low-frequency regimes.

Kinks are localized transitions between topologically distinct ground states and play a central role in systems from condensed matter to cosmology. While acoustic wave packets (here defined as small-amplitude mechanical waves, sometimes referred to as phonons) have been predicted to drive kink motion deterministically, experimental evidence has been elusive, with only stochastic motion from thermal phonons or quasi-static loading observed. This is largely due to the discrete nature of real materials, where the Peierls-Nabarro (PN) barrier hinders controlled phonon-kink interactions. Here, we report experimental observation of acoustic-wave-mediated control and generation of mechanical kinks in a topological metamaterial, which eliminates the PN barrier by supporting a zero-energy kink. We also computationally reveal the dynamics of acoustic wave packet interplay with highly discrete kinks, including long-duration motion and a continuous family of internal modes-features absent in conventional discrete nonlinear systems. Our results enable remote kink control, with potential applications in material stiffness tuning, shape morphing, locomotion, and robust signal transmission.

Mosquito-borne disease and nuisance biting from mosquitoes have severe health and economic consequences. Conventional fabrics are typically not effective at providing protection against mosquito bites, and fabrics treated with repellents and/or insecticides are limited by rising insecticide resistance, risk of significant dermatologic and neurologic side effects, and decreased efficacy with washing and time. The goal of this study was to identify commercially available, repellent/insecticide-free, comfortable fabrics that block bites from three genera of mosquitoes that are known to transmit dangerous infectious diseases with widespread distribution: Aedes, Anopheles, and Culex. To do this, we evaluated fabrics from Ripstop By the Roll LLC in a step-wise series of mouse blood-feeding and behavioral bioassays. Out of 88 fabrics, 53 were found to be blood-feed-proof. These fabrics were more likely to have a higher areal weight density (AWD) and a polyurethane coating than blood-feed-susceptible fabrics. Of the six most comfortable fabrics by subjective hand-feel testing, five were definitively bite-proof during behavioral bioassays. These five fabrics varied substantially in AWD, thickness, finish/coating, and fiber pattern. None of them had a polyurethane coating. Three of them were breathable, making them appropriate for active-wear clothing. Overall, the bite-proof fabrics identified in this study have the potential to significantly reduce mosquito biting and the transmission of mosquito-borne diseases.

The spatial and temporal control of material properties at a distance has yielded many unique innovations including photo-patterning, 3D-printing, and architected material design. To date, most of these innovations have relied on light, heat, sound, or electric current as stimuli for controlling the material properties. Here, we demonstrate that an electric field can induce chemical reactions and subsequent polymerization in composites via piezoelectrically-mediated transduction. The response to an electric field rather than through direct contact with an electrode is mediated by a nanoparticle transducer, i.e., piezoelectric ZnO, which mediates reactions between thiol and alkene monomers, resulting in tunable moduli as a function of voltage, time, and the frequency of the applied AC power. The reactivity of the mixture and the modulus of a naïve material containing these elements can be programmed based on the distribution of the electric field strength. This programmability results in multi-stiffness gels. Additionally, the system can be adjusted for the formation of an electro-adhesive. This simple and generalizable design opens avenues for facile application in adaptive damping and variable-rigidity materials, adhesive, soft robotics, and potentially tissue engineering.

Abstract Acoustic nonreciprocity has received significant interest, particularly in the context of enabling logic devices. One way to break reciprocity is through strategic spatiotemporal modulation of a material’s properties. In this work, we propose and analytically, computationally, and experimentally explore a concept composed of a quasi-one-dimensional nonlinear system, where the shear stiffness depends on longitudinal strain, with the aim that nonreciprocity of transverse–rotational waves could be enabled by the simultaneous propagation of a longitudinal wave injected from the boundary. Such an approach should require less computational overhead in contrast to systems wherein spatiotemporal modulation is accomplished by active control distributed throughout the material, and potentially enable scaling to smaller system sizes and higher frequencies. While good agreement, showing significant nonreciprocity, is found between our analytical predictions and our reduced-order, discrete element model (DEM) simulations, our higher fidelity, finite element model (FEM) simulations, and experiments do not show the same. We suggest that this qualitative difference is due to mechanical instability of the chain, which is not present in either DEM simulations or the analytical model. While providing a theoretical proposal for an all-acoustic spatiotemporally modulated nonreciprocal system, this work also identifies a critical limitation, namely, that of instability, which should be addressed in future related concepts.

Imaging science is a critical enabler of revolutionary scientific advances across disciplines. However, current imaging technologies face prohibitive trade-offs in resolution, penetration depth and experimental complexity. Here, we introduce new classes of micro- and nanostructured photonic surfaces which scale down and enhance light-matter interactions, to overcome existing challenges in imaging science in a miniaturized, on-chip format. We introduce “acoustoplasmonic metasurfaces” to enable tunable acoustic wavefront shaping with polarized light. The proposed acoustoplasmonic metasurfaces merge the physics of light and sound in previously unexplored ways, opening new avenues to harness wave-matter interactions. Future applications of acoustoplasmonic metasurfaces include on-chip imaging with simultaneously high spatial resolution and penetration depth, which can enable societally relevant applications ranging from biomedicine to industrial materials, to environmental science.

Plasmonic metasurfaces describe a class of optical metasurfaces composed of resonant plasmonic nanoparticles, which exhibit strong concentration, scattering, and absorption of electromagnetic energy at subwavelength scales. While their absorptive properties can be a hindrance in all-optical applications, they are advantageous for electromagnetic-to-mechanical energy conversion. This phenomenon is leveraged in acoustoplasmonic metasurfaces, in which incident light excites an acoustic wave. We analytically and numerically examine this acoustic wavefront actuation, which involves the coupling of optical, thermoelastic, and acoustic mechanisms for gold nanospheres. We furthermore demonstrate how nanoparticle anisotropy and relative nanoparticle arrangement enables polarization-tunable acoustic wavefront shaping. The foundational understanding of these mechanisms enables rationally designed wavefront shaping via optimization algorithms. We finally turn our attention towards the experimental realization of these analytical and numerical studies, namely tunable acoustic wavefront actuation using polarized light.

The design of specified nonlinear mechanical responses into a structure or material is a highly sought after capability, with significant potential impacts in areas such as wave tailoring in metamaterials, impact mitigation, soft robotics, and biomedicine. Here, we present a topology optimization approach to design two-dimensional structures for desired uniaxial polynomial nonlinear behavior, wherein we formulate the objective function to match nonlinear coefficient ratios, such that the linear stiffness is decoupled from the desired nonlinearity of the response. We suggest that such linear stiffness decoupling can help aid convergence for problems with fixed, but poorly matched, constituent materials and design volumes. This benefit can be understood by considering, if large absolute force values and stiffnesses are targeted, thicker structures with less open space generally result. Such high volume ratio structures reduce the kinematic freedom (available to, e.g. , long thin structures) which is needed for strong geometrically nonlinear responses. We show designs achieved using this approach that match a range of qualitatively different polynomial behaviors with high precision, which are of interest, in particular, within the domain of dynamical systems where nonlinear elasticity of relatively simple polynomial forms can confer greater analytical tractability.

Optical and acoustic metasurfaces are two-dimensional arrays of subwavelength elements that locally modulate or phase shift incident waves. Acoustoplasmonic metasurfaces combine the physics of light and sound, producing acoustic wavefronts in response to optical stimuli. Herein, we present a computational inverse acoustoplasmonic metasurface design algorithm for desired optically generated acoustic wave fields. We consider gold nanoparticles producing spherical acoustic waves in water, and the resulting acoustic wave propagation along the plane containing the nanoparticle array. We demonstrate how our algorithm can be used to design metasurfaces that can be used to achieve complex acoustic wave fields. This includes the design of a single metasurface that produces acoustic wave fields mimicking two different Morse code patterns upon stimulation with two orthogonal polarization states of light. This work provides a tool for the design of complex optically generated acoustic wavefronts, enabling functionality beyond what would be achievable with off-optical-resonance optoacoustic excitation.

Passive wave transformation via nonlinearity is ubiquitous in settings from acoustics to optics and electromagnetics. It is well known that different nonlinearities yield different effects on propagating signals, which raises the question of "what precise nonlinearity is the best for a given wave tailoring application?" In this work, considering a one-dimensional spring-mass chain connected by polynomial springs (a variant of the Fermi-Pasta-Ulam-Tsingou system), we introduce a bilevel inverse design method which couples the shape optimization of structures for tailored constitutive responses with reduced-order nonlinear dynamical inverse design. We apply it to two qualitatively distinct problems-minimization of peak transmitted kinetic energy from impact, and pulse shape transformation-demonstrating our method's breadth of applicability. For the impact problem, we obtain two fundamental insights. First, small differences in nonlinearity can drastically change the dynamic response of the system, from severely under- to outperforming a comparative linear system. Second, the oft-used strategy of impact mitigation via "energy locking" bistability can be significantly outperformed by our optimal nonlinearity. We validate this case with impact experiments and find excellent agreement. This study establishes a framework for broader passive nonlinear mechanical wave tailoring material design, with applications to computing, signal processing, shock mitigation, and autonomous materials.

The spatial and temporal control of material properties at a distance has yielded many unique innovations including photo-patterning, 3D-printing, and architected material design. To date, most of these innovations have relied on light, heat, sound, or electric current as stimuli for controlling the material properties. Here, we demonstrate that an electric field can induce chemical reactions and subsequent polymerization in composites via piezoelectrically-mediated transduction. The response to an electric field rather than through direct contact with an electrode is mediated by a nanoparticle transducer, i.e., piezoelectric ZnO, which mediates reactions between thiol and alkene monomers, resulting in tunable moduli as a function of voltage, time, and the frequency of the applied AC power. The reactivity of the mixture and the modulus of a naïve material containing these elements can be programmed based on the distribution of the electric field strength. This programmability results in multi-stiffness gels. Additionally, the system can be adjusted for the formation of an electro-adhesive. This simple and generalizable design opens new avenues for facile application in adaptive damping and variable-rigidity materials, adhesive, soft robotics, and potentially tissue engineering.

Unidirectional and backscattering-free propagation of sound waves is of fundamental interest in physics and highly sought-after in engineering. Current strategies utilize topologically protected chiral edge modes in bandgaps, or complex mechanisms involving active constituents or nonlinearity. Here, we propose a new class of passive, linear, one-way edge states based on spin-momentum locking of Rayleigh waves in two-dimensional media in the limit of vanishing bulk to shear modulus ratio, which provides perfect unidirectional and backscattering-free edge propagation that is immune to any edge roughness and has no limitation on its frequency (instead of residing in gaps between bulk bands). We further show that such modes are characterized by a new topological winding number that protects the linear momentum of the wave along the edge. These passive and backscattering-free edge waves have the potential to enable a new class of phononic devices in the form of lattices or continua that work in previously inaccessible frequency ranges.

Kinks are localized transitions between distinct ground states that are associated with a topological charge, significant in fields ranging from condensed matter to cosmology. Theoretical and computational studies have shown that phonons can interact with mechanical kinks to trigger their motion, offering a pathway for kink control. However, the discreteness of most kink-supporting systems introduces a Peierls-Nabarro (PN) barrier, requiring extra energy for kinks to move locally. Here, we report the first experimental observation of mechanical kink control and generation via phonons. To achieve this, we create an elastically-coupled realization of the Kane-Lubensky chain model, which supports a single, topologically protected kink that requires zero energy to deform quasi-statically, resulting in a zero PN barrier. In addition to finding strong agreement between our experiments and numerical simulations, we also numerically observe unique kink dynamics distinct from other nonlinear discrete systems, including continuously smoothly varying set of kink solutions between the neighboring onsite-centered kinks (and associated “shape” mode evolution) and long-duration kink motion, the latter of which has important implications for kink control. Given the topological polarization of our system and the associated features, this work has implications for remote material stiffness control, locomotion, shape-shifting materials, as well as robust signal processing and transmission.

Bistable mechanical metamaterials have shown promise for mitigating the harmful consequences of impact by converting kinetic energy into stored strain energy, offering an alternative and potentially synergistic approach to conventional methods of attenuating energy transmission. In this work, we numerically study the dynamic response of a one-dimensional bistable metamaterial struck by a high speed impactor (where the impactor velocity is commensurate with the sound speed), using the peak kinetic energy experienced at midpoint of the metamaterial compared to that in an otherwise identical linear system as our performance metric. We make five key findings: 1) The bistable material can counter-intuitively perform better (to nearly 1000x better than the linear system) as the viscosity decreases (but remains finite), but only when sufficiently fine discretization has been reached (i.e. the system approaches sufficiently close to the continuum limit); 2) This discretization threshold is sharp, and depends on the viscosity present; 3) The bistable materials can also perform significantly worse than linear systems (for low discretization and viscosity or zero viscosity); 4) The dependence on discretization stems from the partition of energy into trains of solitary waves that have pulse lengths proportional to the unit cell size, where, with intersite viscosity, the solitary wave trains induce high velocity gradients and thus enhanced damping compared to linear, and low-unit-cell-number bistable, materials; and 5) When sufficiently fine discretization has been reached at low viscosities, the bistable system outperforms the linear one for a wide range of impactor conditions. The first point is particularly important, as it shows the existence of a nonlinear dynamical size effect, where, given a protective layer of some thickness...

Unidirectional and backscattering-free propagation of sound waves is of fundamental interest in physics and highly sought-after in engineering. Current strategies utilize topologically protected chiral edge modes in bandgaps, or complex mechanisms involving active constituents or nonlinearity. Here we propose passive, linear, one-way edge states based on spin-momentum locking of Rayleigh waves in two-dimensional media in the limit of vanishing bulk to shear modulus ratio, which provides perfect unidirectional and backscattering-free edge propagation that is immune to any edge roughness and has no limitation on its frequency (instead of residing in gaps between bulk bands). We further show that such modes are characterized by a topological winding number that protects the linear momentum of the wave along the edge. These passive and backscattering-free edge waves have the potential to enable phononic devices in the form of lattices or continua that work in previously inaccessible frequency ranges.

Kinks (or domain walls) are localized transitions between distinct ground states associated with a topological invariant, and are central to many phenomena across physics, from condensed matter to cosmology. While phonon (i.e., small-amplitude vibration) wave packets have been theorized to deterministically interact with kinks and initiate their movement, this interaction has remained elusive in experiments, where only uncontrollable stochastic kink motion generated by thermal phonons or dislocation glide by low-frequency quasi-static loading have been observed. This is partly because all physical systems that support kinks are, at some level, discrete, making deterministic phonon control of kinks extremely challenging due to the existence of Peierls-Nabarro (PN) barrier. Here, we demonstrate, for the first time, experimental observation of phonon-mediated control and generation of mechanical kinks, which we enable using a topological metamaterial that constitutes an elastic realization of the Kane-Lubensky chain model. Our metamaterial overcomes the PN barrier by supporting a single, topologically protected kink that requires zero energy to form and move. Using simulations that show close agreement with our experimental observations, we also reveal unique dynamics of phonon interplay with highly discrete kinks, including long-duration motion and a continuous family of internal modes, features absent in other discrete nonlinear systems. This work introduces a new paradigm for topological kink control, with potential applications in material stiffness tuning, shape morphing, locomotion, and robust signal transmission.

Plasmonic nanoparticles exhibit strong optical scattering and absorption due to enhanced coupling to incident electromagnetic waves, while their efficient photothermal heating enables effective conversion of electromagnetic to mechanical energy. In this work, we put forward a theoretical framework for acoustoplasmonics, where plasmonic nanoparticles control the acoustic wavefront with light. We model the coupled optical, thermoelastic and acoustic mechanisms for gold nanospheres (AuNSs) and nanoellipsoids (AuNEs), and find that each physical mechanism entails a distinct toolbox of parameters, which can be tailored for effective acoustoplasmonic design. Simple analytical studies are performed for AuNSs, both validating numerical models and enabling quasi-analytical wavefront shaping under long laser pulse durations. AuNEs introduce optical anisotropy, and we numerically demonstrate that the polarization-dependent optical absorption in AuNEs can lead to selective photoexcitation and subsequently polarization-tunable acoustic wave generation. Moreover, we investigate the varying acoustoplasmonic frequency regimes, where optical resonance arises due to electromagnetic frequency, while acoustic resonance relates to laser pulse duration. We demonstrate proof-of-concept acoustoplasmonic metasurface designs using these mechanisms for tunable acoustic wavefront shaping in the form of lensing and beam steering. We suggest that future acoustoplasmonic systems, optimized using the physical mechanisms discussed here, will find use in a variety of applications, including miniaturized ultrasonic imaging and high-frequency signal processing.

Natural design and fabrication strategies have long served as a source of inspiration for novel materials with enhanced properties. Less investigated is the prospect of leveraging the complexity of readily available, naturally occurring micro-/nanostructures as platforms for investigating functional materials. In the field of phononics, exploiting structural biocomposites is gaining traction; but finding natural phononic structures that interact with ultra- and hypersonic acoustic waves remains an open quest. In this context, the phononic behavior of natural Nacre, a biocomposite often looked at for inspiration due to its superlattice-like architecture of alternating organic and inorganic phases, is here characterized. To such end, a combination of non-contact pump-probe laser ultrasonics techniques and Brillouin spectroscopy are employed to interrogate Nacre's hierarchical structure at the micro- and nanoscale and measure its phononic dispersion behavior in the MHz and GHz range. It is found that for wavelengths longer than the brick-and-mortar characteristic length, Nacre behaves as a dispersionless medium with effective transversely isotropic properties; but as the wavelengths become comparable to its structural periodicity an involved phononic spectrum arises which challenges the notion of a perfectly periodic, high mechanical-contrast biocomposite.

Metasurfaces are composed of sub-wavelength periodically arranged resonant structures that can manipulate wave-matter interactions in manners not observed in nature. All-optical and all-acoustic metasurfaces have separately demonstrated versatile capabilities such as lensing, beam steering or wavefront control. Here, we study a new class of acoustoplasmonic metasurfaces. By combining the physics of light and sound in previously unexplored ways, this platform enables entirely new avenues to harness the power of wave-matter interactions. This work paves the way toward versatile societal imaging applications ranging from environmental science to biomedical devices or industrial imaging.

Optical and acoustic metasurfaces have been extensively studied for wavefront shaping, including lensing, beam steering, and holography. This work aims to explore a new field of acoustoplasmonic metasurfaces that utilize the photoacoustic effect in gold nanoparticles to generate high-frequency acoustic waves via optical excitation. We leverage the extreme polarization-dependence of absorption efficiency in nanoellipsoids to introduce acoustic wavefront tunability, which opens the door to applications in super-resolution acoustic imaging.

Bistable mechanical metamaterials have shown promise for mitigating the harmful consequences of impact by converting kinetic energy into stored strain energy, offering an alternative and potentially synergistic approach to conventional methods of attenuating energy transmission. In this work, we numerically study the dynamic response of a one-dimensional bistable metamaterial struck by a high speed impactor (where the impactor velocity is commensurate with the sound speed), using the peak kinetic energy experienced at midpoint of the metamaterial compared to that in an otherwise identical linear system as our performance metric. We make five key findings: 1) The bistable material can counter-intuitively perform better (to nearly 1000x better than the linear system) as the viscosity decreases (but remains finite), but only when sufficiently fine discretization has been reached (i.e. the system approaches sufficiently close to the continuum limit); 2) This discretization threshold is sharp, and depends on the viscosity present; 3) The bistable materials can also perform significantly worse than linear systems (for low discretization and viscosity or zero viscosity); 4) The dependence on discretization stems from the partition of energy into trains of solitary waves that have pulse lengths proportional to the unit cell size, where, with intersite viscosity, the solitary wave trains induce high velocity gradients and thus enhanced damping compared to linear, and low-unit-cell-number bistable, materials; and 5) When sufficiently fine discretization has been reached at low viscosities, the bistable system outperforms the linear one for a wide range of impactor conditions. The first point is particularly important, as it shows the existence of a nonlinear dynamical size effect, where, given a protective layer of some thickness...

An updated analysis method for laser-induced transient grating spectroscopy measurements of near-surface thermal diffusivity using gigahertz frequency instrumentation is presented. Considering the particular application of characterizing materials with heavy ion damage, prior analysis methods typically assume a very short excitation pulse relative to the relaxation time of the response signal. For longer pulse durations, this can be an increasingly poor assumption with decreasing probing depth. This work presents the application of convolution to update the previous analysis method. As a case study, a 400 ps excitation laser pulse on tungsten with transient grating wavelengths of 1.33–20.0 μm, corresponding to characteristic decay times of 0.633–143 ns, is analyzed. Transient grating response curves from numerical simulations and experimental transient grating spectroscopy data are fit using each analysis method, with and without convolution, and compared. Considerations with regard to the data collection instruments are also detailed.

The thermal transport and elastic properties of materials are often measured using the laser-induced transient grating spectroscopy (TGS) technique. The analysis of the TGS signal usually involves fitting well-known expressions, derived assuming the limiting cases of opaque or transparent materials, to the measured data. In this paper, the system of thermoelastic equations is analytically solved for an isotropic homogeneous material assuming finite laser penetration depth, which is an important consideration when the penetration depth is on the order of the acoustic wavelength. The need to use such a solution is discussed and compared to the expression for opaque material. The solution is benchmarked against TGS signal measured on single-crystal silicon with {100} surface orientation and is found to significantly improve the accuracy of the calculated thermal diffusivity as compared to using the expression for opaque material.

Passive transformation of waves via nonlinear systems is ubiquitous in settings ranging from acoustics to optics and electromagnetics. Passivity is of particular importance for responding rapidly to stimuli and nonlinearity enormously expands signal transformability compared to linear systems due to the breaking of superposition. It is well known that different types of nonlinearity yield vastly different effects on propagating signals, which raises the question of ``what precise nonlinearity is the best for a given wave tailoring application?'' Considering a one-dimensional spring-mass chain as a testbed, we couple the shape optimization of structures for tailored nonlinear constitutive responses with reduced-order nonlinear dynamical inverse design. Using minimization of peak kinetic energy transmission from impact as a case study, we identify ideal nonlinear constitutive responses and the geometries needed to achieve them. As part of this, we show the large sensitivity of this metric to small changes in nonlinearity, and thus the need for high precision, free-form nonlinearity tailoring. We validate our predictions using impact experiments in a chain of nonlinear springs and masses. This work sets the foundation for broader passive nonlinear mechanical wave tailoring material design, with applications to computing, signal processing, shock mitigation, and autonomous materials.

Unidirectional and backscattering-free propagation of sound waves is of fundamental interest in physics and highly sought-after in engineering. Current strategies utilize topologically protected chiral edge modes in bandgaps or complex mechanisms involving active constituents or nonlinearity. Here, we propose a new class of passive, linear, one-way edge states based on spin-momentum locking of Rayleigh waves in two-dimensional media in the limit of vanishing bulk modulus, which provides 100% unidirectional and backscattering-free edge propagation immune to any edge roughness at a broad range of frequencies instead of residing in gaps between bulk bands. We further show that such modes are characterized by a new topological winding number that is analogous to discrete angular momentum eigenvalues in quantum mechanics. These passive and backscattering-free edge waves have the potential to enable a new class of phononic devices in the form of lattices or continua that work in previously inaccessible frequency ranges.

Topological mechanical metamaterials have enabled new ways to control stress and deformation propagation. Exemplified by Maxwell lattices, they have been studied extensively using a linearized formalism. Herein, we study a two-dimensional topological Maxwell lattice by exploring its large deformation quasi-static response using geometric numerical simulations and experiments. We observe spatial nonlinear wave-like phenomena such as harmonic generation, localized domain switching, amplification-enhanced frequency conversion, and solitary waves. We further map our linearized, homogenized system to a non-Hermitian, nonreciprocal, one-dimensional wave equation, revealing an equivalence between the deformation fields of two-dimensional topological Maxwell lattices and nonlinear dynamical phenomena in one-dimensional active systems. Our study opens a regime for topological mechanical metamaterials and expands their application potential in areas including adaptive and smart materials and mechanical logic, wherein concepts from nonlinear dynamics may be used to create intricate, tailored spatial deformation and stress fields greatly transcending conventional elasticity.

the use of such a relatively computationally inexpensive coarse-grained molecular dynamics model, which may be of further value to application areas including soft robotics, bio-mimicking devices, artificial muscles, and adaptive materials.

Accurate modeling and prediction of damage induced by dynamic loading in materials have long proved to be a difficult task. Examination of postmortem recovered samples cannot capture the time-dependent evolution of void nucleation and growth, and attempts at analytical models are hindered by the necessity to make simplifying assumptions, because of the lack of high-resolution, in situ, time-resolved experimental data. We use absorption contrast imaging to directly image the time evolution of spall damage in metals at ∼1.6-μm spatial resolution. We observe a dependence of void distribution and size on time and microstructure. The insights gained from these data can be used to validate and improve dynamic damage prediction models, which have the potential to lead to the design of superior damage-resistant materials.

Many active materials have properties that make them challenging to design with or limit their utility, including having combinations of nonlinear, viscous, slow, or small responses. In this work, we show that by designing metamaterial unit cells that have separately active and, specifically geometrically nonlinear, passive elements, many of these drawbacks can be mitigated and enhanced performance enabled (specifically, large, rapid, and resettable shape change). This separation between active and passive elements is advantageous, in that the complex active materials can be assigned only where needed, with the simplest geometries and functions. The use of geometric nonlinearity in passive elements is helpful as it provides mechanisms for augmented performance and further simplifies the role of the active elements. While all elements of the microstructural motifs used herein have been previously seen elsewhere, we suggest that the contribution provided here is a conceptual design approach that may be used more broadly for enabling new types of high-performing, stimuli-responsive metamaterials.

Topological physics has revolutionized materials science, introducing topological phases of matter in diverse settings ranging from quantum to photonic and phononic systems. Herein, we present a family of topological systems, which we term "strain topological metamaterials", whose topological properties are hidden and unveiled only under higher-order (strain) coordinate transformations. We firstly show that the canonical mass dimer, a model that can describe various settings such as electrical circuits and optics, among others, belongs to this family where strain coordinates reveal a topological nontriviality for the edge states at free boundaries. Subsequently, we introduce a mechanical analog of the Majorana-supporting Kitaev chain, which supports topological edge states for both fixed and free boundaries within the proposed framework. Thus, our findings not only extend the way topological edge states are identified, but also promote the fabrication of novel topological metamaterials in various fields, with more complex, tailored boundaries.

An experiential approach to improve learning outcomes in the area of ethics training for mechanical and aerospace engineering upper-class undergraduates has been previously described [1]. Due to the initial positive outcomes and feedback from students, an ethics module comprised of three interactive, experiential exercises introduced into a core-curriculum laboratory class was deployed the following academic year. In this iteration as part of the module, upper-class students from aerospace engineering as well as mechanical engineering were pre-divided into groups of four and asked to select from a curated selection of case studies relevant to their major. They were then asked to create a compelling, three-minute video in which the roles of client, manager, engineer, and any other role deemed relevant, embodied the complexity of making difficult ethical decisions in the selected case study. The assumption in the past had been that students simply needed to learn how to comprehend how stakeholders might perceive a problem from multiple perspectives, and in the communication of these views develop a deeper understanding that would enable them to make sound ethical decisions. While this may be a critical part in teaching ethical decision making, it is clear that another, perhaps more fundamental enabling aspect is in establishing a sense of trust that encouraged deeper reflection and sharing amongst the students which provides the foundation for the subsequent empathetic understanding and communication. Contrary to previous assumptions about the primacy of communication as foundational for the establishment of ethically sound decision-making, this paper discusses the importance of creating an environment of trust wherein leadership in equitable and ethical decision-making can be promoted.

Imaging techniques with subdiffraction-limited spatial resolutions are highly desired for a deeper understanding of subcellular systems. Optical imaging enables high resolution under 200 nm while the visible light penetration depth is limited to merely 2 mm. Ultrasound images achieve two orders of magnitude lower resolutions but can penetrate two orders of magnitude deeper into a medium than optical images. This work combines the strengths of optical and acoustic imaging techniques through AuNP-based metasurfaces utilizing the photoacoustic effect that gold exhibits. Our novel imaging technique can simultaneously achieve high resolution and deep penetration depths without any destruction of media.

Using an alternative mechanism to dissipation or scattering, bistable structures and mechanical metamaterials have shown promise for mitigating the detrimental effects of impact by reversibly locking energy into strained material. Herein, we extend prior works on impact absorption via bistable metamaterials to computationally explore the dependence of kinetic energy transmission on the velocity and mass of the impactor, with strain rates exceeding 102 s−1. We observe a large dependence on both impactor parameters, ranging from significantly better to worse performance than a comparative linear material. We then correlate the variability in performance to solitary wave formation in the system and give analytical estimates of idealized energy absorption capacity under dynamic loading. In addition, we find a significant dependence on damping accompanied by a qualitative difference in solitary wave propagation within the system. The complex dynamics revealed in this study offer potential future guidance for the application of bistable metamaterials to applications including human and engineered system shock and impact protection devices.