3D printing opens new avenues for designing and fabricating architected materials in a scalable manner. Specifically, we fabricate tailored architectures by direct writing of soft, hard, and composite inks. Using digital assembly, we programmably encode the mechanical responses of interest, including bending, snap-through instabilities, specific stiffness, and Poisson&’s ratio. This talk will highlight multiple examples of novel architectures and their resulting mechanical properties created by this approach.

In this study, we suggest a new methodology to construct a 3D metal/elastomer hybrid microstructure by stiffness patterning and folding instability. Thin, layered elastomer undergoes the unstable process in the presence of the compression, creating the wrinkle or fold structure. Here, we adopted stiffness patterning method on the surface of the elastomer to control the position of fold structure. Subject to a compressive surface strain, the stiffness-patterned substrate could produce selective-deformation depending on substrate stiffness. Thus the folded structure appears on the softer part of the substrate. Using this approach, we constructed 3D metal-elastomer hybrid microstructure by site-selective folding instability combined with a wet transfer method. The folded metal-elastomer hybrid microstructure is reversibly stretched and compressed to large levels (200 %) of strain without damage in the metal structure. Moreover, our method opens the way to provide a new kind of microfabrication design like optical film or mechanical sensor.

Cellular solids are instrumental in creating lightweight, strong, and damage-tolerant engineering materials. By extending their feature size down to the nanoscale, we are able to simultaneously exploit the architecture and size effects to substantially enhance the structural integrity of architected meta-materials. We show that hollow-tube alumina nanolattices of 3D kagome geometry with and without pre-fabricated notches always fail at the same load when the ratio of notch length (a) to sample width (w) is no greater than 1/3, with no correlation between failure occurring at or away from the notch. For notches with (a/w) > 1/3, the samples fail at lower peak loads and this is attributed to the increased compliance as fewer unit cells span the un-notched region. Finite element simulations of the kagome tension samples show that the failure is governed by tensile loading for (a/w) < 1/3 but as (a/w) increases, bending begins to play a significant role in the failure. This work explores the flaw sensitivity of hollow alumina kagome nanolattices in tension, using experiments and simulations, and demonstrates that the discrete-continuum duality of architected structural meta-materials gives rise to their flaw insensitivity and fracture tolerance even when made entirely of intrinsically brittle materials.

Sacrificial templating offers a versatile method of synthesizing new materials with hierarchically porous structures for use as catalysts and absorbents. This process facilitates precise tunability of pore size in order to produce materials with rationally designed pore structures spanning all porosity regimes (macroporous, mesoporous, and microporous). Such materials offer the ability to selectively engineer a catalyst or absorbent in order to obtain desirable transport properties during chemical processing. This work demonstrates the ability to produce three dimensionally ordered mesoporous materials (3DOm) including silica&’s, carbons, and transition metal oxides using various colloidal templating techniques. These materials demonstrated the ability to tune surface area and pore size via templating; while simultaneously stabilizing metastable oxide phases such as a-TiO₂ and t-ZrO₂ at elevated temperatures. The versatility of these hierarchically porous materials is demonstrated via the selective separation of sugars and the photocatalytic decomposition of methylene blue dye. To characterize the samples, precursor solutions were analyzed using DLS and SAXS prior to templating; while templated materials were characterized using N₂ and Ar adsorption, SEM, HRTEM, XRD, SAXS, and XPS. The discussion will conclude with a review of these findings in order to highlight the appropriate selection of chemical precursor&’s, processing conditions, and potential pitfalls to consider when synthesizing future materials via templating.

3D printing opens new avenues for designing and fabricating architected materials in a scalable manner. We use a 3D extrusion-based printing method, known as direct ink writing, to fabricate soft, reversibly-deformable architected materials. Specifically, silicone-based inks with tailored rheology are patterned into structures comprised of precise beam geometries. The aspect ratio and the tilting angle affect the mechanical response of the beams, which can include such phenomena as snap-through instabilities and multistability, leading to nonlinear wave propagation. Beam geometries can be programmably tuned to encode spatially-varying mechanical responses. Due to the soft architecture, the static and dynamic response of the architected material can be further tuned by applying static deformation. This talk will highlight multiple examples of novel architectures and mechanical responses created by this approach.

Recent advances in topological optimization methodologies for design of internal material architecture, coupled with the emergence of micro- and nanoscale fabrication processes, 3D imaging, and micron scale testing methodologies, now make it possible to design, fabricate, and characterize lattice materials with unprecedented control. This talk will describe a collaborative effort that employs scalable 3D textile manufacturing, location specific joining, and vapor phase alloying to produce metallic lattices with a wide range of internal architectures, alloy compositions, and mechanical and functional properties. Topology optimization allows properties to be decoupled and tailored for specific applications. Dramatic enhancements in permeability have been balanced with modest reductions in stiffness, and are being used to develop heat exchanger materials with high thermal transport, low impedance, low thermal gradients and high temperature strength. In a parallel effort, architectural designs to maximize both structural resonance and inter-wire friction are also being employed to develop metallic lattices capable of mechanical damping at elevated temperatures. These examples will be used to highlight the benefits to be gained by the design, manufacturing and characterization of metallic lattice materials with a wide range of tailorable properties.

3D printing has opened the doors to design freedom such that just about anything that can be imagined and sketched up in CAD can be fabricated from plastics, metals and even ceramics. The plastic systems which have been around for more than 25 years, have been limited by the fact that it is difficult to incorporate reinforcement into the resins so mechanical properties are generally limited to neat resin stiffness and strengths. Additionally the layer by layer process of making parts often leads to weakness in the laminations. Metal AM parts on the other had have mechanical properties similar to wrought metal properties of the same composition but it is generally very expensive to build and finish metal AM parts limiting their application.
Electroplating additive manufactured organic resin parts can be a great complement to SLA, SLA, FDM and other 3D printing methods. Coating of copper and nickel can provide metallic properties such as heat dissipation, electromagnetic and optical reflectivity, barriers against harsh environments, improve thermal stability as well as stiffen and strengthen parts in a predictable way. This presentation will review the mechanics and methodology for applying the coatings for different purposes as well as simple design modifications, easily done with print on demand additive manufacturing, that facilitate making functional accurate metal clad plastic parts. Additionally, the issues encountered in working with the different AM systems will be covered along with how these can be dealt with and processed successfully.

The collective physical properties of nanoparticle (NP) assemblies depend strongly on interparticle distance. Controlled tuning of interparticle spacings therefore offers the possibility to optimize the response of NP solids for applications including optical, magnetic and electronic devices.
In this series of projects, a variety of lipophilic, highly flexible, dendritic ligands (generations 0 to 4) was designed to bind colloidal nanocrystals and induce self-assembly properties. After ligand exchange, by controlling the solvent evaporation rate, the corresponding dendron-capped nanoparticle hybrids were found to self-organize into hexagonal close-packed (hcp) superlattices. The interparticular spacing can be progressively varied from 2.2 to 6.3 nm with increasing the dendritic generation, covering a range that is intermediate between commercial ligands and DNA-based ligand shells.
Dual mixtures of dendronized hybrids resulted in unprecedented superlatices (where both components have same size inorganic core, but different dendritic covering) which are isostructural with NaZn₁₃ and CaCu₅ crystals.
The synthetic and self-assembly details as well as latest results will be discussed.

Non-close packed colloidal lattices made by self-assembly can be used to fabricate a variety of large-scale nanostructures (honeycomb, nano-pillars) by nanosphere lithography, and devices such as photonic crystals and biosensors [1]. This method has benefits over close-packed arrays due to greater applicability with tunable interparticle distance, and less susceptibility to particle size variation and solution contaminants. Multiple methods exist for self-assembly of close-packed polystyrene microspheres [2], but achieving control over microsphere spacing presently requires chemical and/or mechanical modifications of the substrate or the particles [1, 3]. We present an ultrasonic mist deposition approach that leads to robust, large-scale ordered structures of microspheres, with interparticle spacing above four particle diameters. Starting with a solvent-microsphere colloid, a piezoelectric element is used to generate a mist that passes through a gravity-buoyancy barrier to preferentially select mist particles containing individual microspheres before being transported onto a water surface. Upon landing on the water, at specific interparticle spacing an isotropic pair potential between the microspheres is established due to the combination of zeta potential, capillary, and surface tension forces at the water-air interface. Unlike the work of [4], the colloidal lattice is formed at a water-air interface and exhibits pair potential interactions without an applied electric field, suggesting a new mechanism for interparticle interaction at the observed spacing. Use of thisnew mist method, as opposed to traditional Langmuir-Blodgett or spin coating methods, enhances the fraction of microspheres deposited at the appropriate spacing to enter the ordered lattice potential well, while enabling straightforward extension to large scale deposition. No surface modifications to the microspheres or substrate are required, furthering the flexibility and ease of this approach. This work was supported by the SUTD-MIT International Design Center.
1. Jian-Tao Zhang, et al. Periodicity-Controlled Two-Dimensional Crystalline Colloidal Arrays. Langmuir vol. 27, 15230-15235 (2011).
2. Junhu Zhang, et al. Colloidal Self-Assembly Meets Nanofabrication: From Two-Dimensional Colloidal Crystals to Nanostructure Arrays. Advanced Materials vol. 22, 4249-4269 (2010).
3. Xiao Li, et al. Modulating Two-Dimensional Non-Close-Packed Colloidal Crystal Arrays by Deformable Soft Lithography. Langmuir vol. 26, 2930-2936 (2010).
4. Ke-Qin Zhang & Xiang Y. Liu. In situ observation of colloidal monolayer nucleation driven by an alternating electric field. Nature vol. 429, 739-743 (2004).

A great deal of effort has been devoted to stack 2D planar nanostructures for the amplification of its own photonic functionality or the achivement of newly derived photonic behavior, such as chirality. In our experiment, we demonstrated to stack 3D nanostructures layer upon layer via repetitive execution of e-beam lithography and 3D assembly of nanoparticles via IAAL (Ion-Assisted Aerosol Lithography).
Our multi-layer 3D nanostructures were fabricated through following 3-step procedures. (1) We generated cross-shaped patterns with the size of 1.5mu;m#8553;1.5mu;m and the interval of 1.5mu;m on the silicon substrate via e-beam lithography. (2) Positively charged metal nanoparticles generated by spark discharge were blown onto the pattered silicon substrates where negative voltage was applied. nanoparticles assembled on the exposed silicon substrate and grew in the lateral directions, due to the nanoscopic electrostatic lens induced by the pre-deposited cations on e-beam resist. (3) 3D nanostructures were sintered and strengthened by e-beam irradiation to endure the spin-coating of e-beam resist for the upper layer patterns. Then, e-beam resist was spin-coated upon the nanostructure of the previous layer and patterns for the next layer which was aligned according to previous layer was fabricated via e-beam lithography. Through repetition of procedure (1)-(3), multi-layer 3D nanostructures were manufactured. Finally, e-beam resist was eradicated through O₂ plasma ashing.
In order to demonstrate the photonic enhancement of multi-layer nanostructures, compared to mono-layer nanostructures, we measured the SERS signals of thiophenol molecueles chemisorbed onto the bi-layer nanostructures and the mono-layer nanostructures. It was observed that the peak intensities of the bi-layer nanostructures were significantly increased, compared to those of the mono-layer nanostructures. The bi-layer nanostructures exhibited 4-5 fold higher peak intensities, compared to the mono-layer nanostructures.

This work examines the effect of corrugated architecture on stress transfer in metal matrix composite materials. Due to the fact that during unbending a corrugation exhibits a transition from bending dominated behaviour towards stretching dominated behaviour it is possible to construct a composite material that transitions from minimal stress transfer initially to large amounts of stress transfer upon further deformation. During the loading of a corrugation reinforced composite, the corrugation will unbend leading to an evolving reinforcement alignment and therefore evolving conditions for the transfer of stress. In this work, the stress transfer in composites with plastically deforming components is analyzed using an adapted shear lag model and Finite Element Modeling. Experimental investigations into strain distribution are also included.

Recently, block copolymers (BCPs) have been investigated extensively for nanolithography applications to provide highly controlled structures with a simple and cost-effective method. Among various structures, vertically aligned pillars are one of the most prominent architectures for the emerging application, such as vertical transistor, light reflection controller, and chemical and biological sensor. Because of the easiness of surface modification, silica nano-pillars are promising candidate to realize these applications. In this presentation, we demonstrate a facile route to fabricate a well ordered vertically aligned silica nano-pillars making the most of self-assembled BCPs.
Polystyrene-block-polydimethylsiloxane (PS-b-PDMS; 31k-b-14.5k) was dissolved in cyclohexane, followed by spin coating on the Si substrate which was coated with PS brush in advance. The solvent annealing was conducted in chloroform vapor within 2 hours. The reactive ion etching (RIE) and the sequential calcination could easily provide silica nanostructures. The morphology of the obtained structures was observed by scanning electron microscope (SEM), whereas the composition was verified by Fourier transform infrared spectroscopy (FT-IR). Grazing-incidence small angle X-ray scattering (GISAXS) measurements were also conducted to investigate the morphological transition as well as the ordering of BCPs during annealing process. With increasing the annealing time, micro phase separation was induced and cylindrical PDMS became the dominant phase, leading to the optimum condition of 1.5 hours. Well aligned nano-pillars could be found throughout the substrate of 1 cm². Over 2 hours, the structure gradually lost the regularity. In addition, we also investigated the effect of the vapor pressure, and drying speed of the solvent on the morphology of the samples. GISAXS measurements well agree the observation of SEM images. Some optical properties derived from fine structure of the fabricated nano-pillars will be presented.

Lizards living in desert have remarkable ability to collect and transport water through their skin to the mouth. The lizards drink water using specialized scaled integuments, semi-tubular capillary system, which consists of honeycomb-like scales as water reservoir, and interscalar channels as water transporter. Inspired from these unique integuments, we suggest that the lizard&’s water drinking system can be major components of open-closed hybrid microfluidic systems. Thus, we utilized a novel controlled fracture technique for a microfluidic system.
Although fractures or cracks are typically difficult to control precisely due to their randomness, crack formation can be controlled under the delicate conditions. In this study, we controlled the stress distribution throughout the surface at the predefined position. As expected, controlled cracks were created from the predefined notches as initiating points. When the extent of tensile strain became increased, the number of cracks formation was also increased. At the 75% of prestretched condition, the cracks were formed completely at almost of prepatterned notches. From these results, we suggested that the controlled fracture was successfully fabricated from the initiating notches and guiding line under the prestrained condition.
By introducing fluorescent dye to visualize the surfaces of fabricated system, we observed that the dye was firstly retained on the crack induced open channels and subsequently absorbed into folded closed channels by the capillary force. For the strains from 45% to 60%, interestingly, there was only dye absorption into folded channels starting from the crack. The dye was detected over all of the closed channels at 75% complete crack formation.
In conclusion, we have presented a simple, but highly reproducible technique to fabricate a bio-inspired hybrid microfluidic system by simply varying the prestrained condition of prepattened elastomer. This hybrid system is collectively applicable to control the fluid flow in the microfluidic system.

Nanostructures can often be viewed as mathematically distinct and measurable because of the repetition in the geometry of their underlying structures. Far from being random, these patterns reflect the order of established scientific rules, and rigorous analogues between mathematical objects and subsets of nanostructures can be made. Under the aegis of physical and chemical laws, nanostructures form discrete and measurable geometric patterns ranging from repeating lattices to complicated polygons. Established work from several areas of pure mathematics (such as graph theory and Ramsey theory) can be used to analyze and predict potential properties from well-defined nanostructures. Specifically, carbon has distinct allotropes that build upon the basic honeycomb-like lattice structure of graphene. Because these allotropes have clear commonalities in terms of geometric properties, this paper details several approaches to the use of graph theory to enumerate structures and properties of nanocarbons. Graph theoretic treatment of the lattice that forms the foundation of graphene is completed, and parameters for obtaining other forms of nanocarbon based on these graph theoretic principles are established. Graph theoretic methods of understanding, analyzing, and predicting behavior of nanostructures are detailed and established as a potentially innovative method of understanding carbon nanostructures.

A combined theoretical and experimental study on α-Ag₂WO₄crystals is presented. Structural and optical properties and the formation of Ag nanoparticles on α-Ag₂WO₄ induced by electron irradiation have been studied. α-Ag₂WO₄ samples was prepared by co-precipitation method. The samples were characterized by X-ray diffraction (XRD), micro-Raman (MR) spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) characterization. Their optical properties were investigated by ultravioletminus;visible (UV-vis) spectroscopy and photoluminescence (PL) measurements at room temperature. Thermodynamic equilibrium shape of the α-Ag₂WO₄crystal is built with the calculated surface energies through a Wulff construction. The (110) and (011) orientation is the dominating surfaces in the Wulff shape. It suggests that the Wulff shape of α-Ag₂WO₄ is closely related to the chemical environment around, and a larger increase in surface energies for the ((010)/ surface rather than those in the other surfaces allows us to find a good agreement between experimental and theoretical morphology. We have revealed a correlation between the exposed morphology and presence of hydroxyl anions, and have provided an explanation to the puzzle that arises from. We believe that these findings can be used as an important tool to the understanding of controlled structural and physical characteristics in nano- and micro-crystals.

This paper describes a method of 3D printing of three-dimensional micro-architectures with embedded functional nanoscale materials such as silicon nanowires and enzymes . A three-dimensional CAD model is first sliced into a series of closely spaced horizontal planes. These two-dimensional image slices are sequentially transmitted to the reflective liquid-crystal-on-silicon chip, which is illuminated with UV light from a light emitting diode array. Each image is projected through a reduction lens onto the surface of the photosensitive resin containing encapsulated nanomaterials. The exposed liquid cures, forming a layer in the shape of the two-dimensional image, and the substrate on which it rests is lowered, reflowing a thin film of liquid over the cured layer. A microfluidic material delivering system is used to flush the first liquid and deliver the second liquid on top of the first layer. The image projection is then repeated with the next image slice forming the subsequent layer.he substrate travels in the z-direction between each build layer, perpendicular to the fluid surface of the carrier fluid bath, and is controlled by a motorized linear translation stage. The unique advantage of 3D printing technique is the capability of 3D printing a large array of multi-materials or materials with graded encapsulated material concentrations and functionalities.

Statement of Purpose: 3D hydrogel cell cultures provide a unique environment for studying cell interactions by more closely mimicking in vivo extracellular matrix. However, many tissues, particularly connective tissue and cartilage, exhibit anisotropic mechanical properties that are not replicated by hydrogels. Recent advances in additive manufacturing combined with visible-light photopatterning and low-toxicity photoinitiators have enabled a new field of additive biomanufacturing for high-resolution spatial patterning of scaffolds. Using these advantages, superposition of microarchitected lattices within the hydrogel can mimic tissue anisotropies, providing new, synthetic environments for cell cultures. Specifically, a microarchitected lattice can confer an orthotropic Poisson's ratio to the surrounding hydrogel.
Methods: The hydrogel constructs consist of two parts: 1) mechanical lattice and 2) hydrogel filler. The mechanical lattice was fabricated using Projection Microstereolithography, a digital mask-based 3D printer. To create 3D structures, digital models are converted to a series of images and projected into a UV-curable resin in coordination with precise movements of the substrate. In this way, 3D structures are built from a sequence of 2D images.
The mechanical lattice was fabricated from UV-cured PEGDA monomer. After the lattice was fabricated, the hydrogel resin was added to the lattice and cured in place using a UV lamp. Prior to photopolymerization, 10um polystyrene beads at a concentration of 1% (w/w) were added to the hydrogel resin for visualization.
Mechanical properties were measured on an Instron 5943 and strain measurements were calculated optically during compression.
Results: The results show a relationship between the microarchitecture and Poisson's ratio of the sample. Polystyrene beads acting as displacement markers throughout the patterned volume will give local displacement values along the imaging plane, generating a spatial displacement map. The microarchitecture alone has been tested and showed an orthotropic Poisson's ratio of up to 1 along a single plane.
Conclusions: Recent advances in additive biomanufacturing enable the creation of hydrogel environments with unique mechanical properties. Hydrogels with embedded microarchitecture can allow control over mechanical properties such as Poisson's ratio, providing new environments for studying cell interactions as well as tissue engineering applications.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
LLNL-ABS-669732

The Interface between neural electrodes and neural tissue plays an important role in neural recording and stimulation. Low impedance electrodes are required for development of high signal-to-noise ratio neural recording devices. To design a low impedance interface, we need to understand the electrical and physical process contributing to the impedance. Conducting polymers (CPs) have been widely used in biomedical application especially neural interfaces owning to (1) their organic nature; (2) both ionic and electronic conductivity; and (3) ability to be functionalized with bioactive molecules. Here we report fabrication of Conducting polymer poly (pyrrole) (PPy) microcups using microsphere templates. PPy-coated poly (lactic-co-glycolic) acid (PLGA) microspheres deposited on gold electrodes could decrease the impedance of electrodes by increasing the effective electrode surface area. To access the electrical properties of PPy microcups, electrochemical impedance spectroscopy (EIS) was employed. An equivalent circuit model including a coating capacitance in parallel with a pore resistance and interface impedance in series was proposed and fitted to EIS experimental measurements. Curve fitting results indicated that the proposed model supported the experimental data with high consistency. In this study, 4wt% PLGA with 2wt% benzyltriethylammonium chloride (BTEAC) with respect to PLGA was dissolve in chloroform and electrosprayed on gold substrate. PLGA microspheres were coated with PPy using electrochemical deposition with 0.5 mA/cm² current density for 8 minutes. EIS measurements were performed by applying an AC sine wave with 10 mV rms amplitude over the frequency range of 1-10⁴ Hz. The equivalent circuit model consisted of several electrical elements such as double layer capacitance, charge transfer resistance, Warburg impedance, coating capacitance and resistance, and solution resistance. In summary, fabrication and measurement techniques with the corresponding equivalent circuit model have been developed to characterize the electrical properties of electrode-electrolyte interface. This proposed model provided a tool for better understanding of complex electrode-electrolyte interface. Furthermore, we demonstrated that the experimental and fitted results are consistent with high accuracy. In future, we aim to improve the conducting polymer microcup fabrication process using the predicted data from our proposed model.

Several hollow hierarchical structures have been synthesized through templating and etch methods. This study focuses on single ligaments of the structures by developing tubes with metal-foam sandwich walls that are designed to resist buckling, a common failure mode for hierarchical lattices. A high-melting point metal is used for the sandwich walls while the foam core is made of a nanoporous metal. Tubes are fabricated by using a series of synthesis methods beginning with the electrodeposition of two metals onto a thermally stable metal tube. The deposited metals are subsequently homogenized to generate a binary alloy and the nanoporous foam is made by dealloying the diffused metals. Finally, a sputtered layer of metal adds the outer sandwich panel. The geometry and porosity of the foam is further modified through heat treatments that coarsen the ligament size of the foam and give control over the available surface area. These structures not only have the potential to improve the deformation behavior of more complex hierarchical structures, but also multi-functionalize them through the extremely high surface area offered by nanoporosity.

An unprecedented deagglomeration and uniform distribution of Al₂O₃ nano-dispersoids in the Aluminum matrix is achieved through a novel two step contact-non contact ultrasonic casting technique that combines stirring, contact type ultrasonication and non contact type ultrasonication. It is known that contact type ultrasonic casting leads to pushing of nano-dispersoids and reagglomeration during solidification. The microstructure consists of three distinguished regions: grain boundary segregation zone; particle depleted zone and particle reinforced zone. Non contact type ultrasonic casting possess less time for complete deagglomeration during solidification. The microstructure consists of mixture of small agglomerates and deagglomeated particles. Unlike in contact type ultrasonic casting, no such definite zones are observed. During this process, engulfing of particles is dominated over pushing and hence grains without particulate reinforced zone are not formed. Thus, both processes in individual provide neither complete deagglomeration nor distribution. Hence in the present work, a novel two step contact-non contact ultrasonication coupled with electrical stirring is introduced for the first time ever. The contact type ultrasonication along with stirring helps in not only effective deagglomeration but also uniform distribution of nano-dispersoids throughout the melt and on the other hand, the non contact type ultrasonication avoids pushing and reagglomeration during solidification. The microstructure achieved during this process contains neither depleted zones and nor (or) least grain boundary segregation. Particles are uniformly distributed in the matrix. The method presented in this work can be promising way of synthesizing bulk MMNCs with highest deagglomeration and highest uniform distribution of nano-dispersoids.

Planar optical microcavities (MCs) are light confining structures employed in optics, photonics, and quantum information processing studies. The efficient and fast tuning of MC resonances can be extremely advantageous in photonics devices and quantum optics experiments. On other hand, surface acoustic waves (SAWs) have emerged in the past few years as a powerful and non-destructive tool to achieve modulation of the optical properties of semiconductor light emitting nanostructures like quantum wells and quantum dots.
In this work, we simulate the dynamic optical response of highly piezoelectric MCs under the modulation induced by SAWs. We design MCs that resonate in the visible/NIR optical range and are grown on sapphire substrates to allow both reflectivity and transmission experiments. We work with structures mainly based on ZnO/SiO2 and LiNbO3/SiO2 double Bragg mirrors (DBRs) which sandwich SiO2 (or polymer) active layers. By placing aluminum interdigital transducers (IDTs) on the samples surfaces, we calculate the modulation of the optical resonances of the MCs induced by the strain and piezoelectric fields of the SAW across the structures. We show how the strong interaction between photons in the cavity and SAW phonons leads to the folding of the dispersion relation of these systems and, consequently, to new optical resonances appearing in the spectrum. These resonances depend strongly on the SAW power which allows for the tuning of the MCs resonances over wide ranges of energies. In this way, we discuss the application of highly piezoelectric MCs in experiments where nanostructures with different transition energies are coupled to different resonances of confined electromagnetic field.

Developing a reliable sealant or sealing system remains one of the top priorities in research on planar solid oxide fuel cell (SOFC) technology. Recent studies on SOFC glass-based sealants have focused on self-healing glass seals, such as those made of compliant alkali-containing silicate glass. However, thermochemical and thermomechanical properties of self-healing alkali-silicate glasses are usually weaker than those of alkaline-earth silicate glasses. One way to overcome such weaknesses is to use fillers to modify glass properties. For example, Al₂O₃ filler can strengthen alkali-silicate glass network structure. The objective of this study is to find suitable fillers for applications in solid oxide fuel cell sealing glass. This was accomplished by adding various amounts of Al₂O₃ and 8 mol % Y₂O₃-stabilized zirconia (8YSZ) powders as fillers to the alkali-containing borosilicate sealing glass prepared in this work. The effect of these fillers on the viscosities, electrical conductivities, and infrared spectroscopy of the sealants were investigated. It was concluded that long-term sealing behavior could be improved by simultaneous addition of Al₂O₃ and 8YSZ fillers.

Recent advancements in manufacturing have provided unprecedented abilities to realize engineered materials with defined pore architectures. This has presented new opportunities in design, with topology optimization (TO) in particular being particularly well-suited to explore this now expanded design space. Topology optimization is a systematic design methodology for optimizing material distribution across a design domain. Material may be (computationally) added or removed freely, meaning architectural connectivity and shape may evolve during the design process. This design freedom enables discovery of new material architectures, often predicted to approach theoretical bounds, but typically comes at the price of reduced manufacturability and increased cost. This talk will discuss the integration of manufacturing into the TO design process so that designed architectures respect manufacturing constraints (and thereby limit re-design) and leverage new manufacturing capabilities. Various manufacturing processes are considered (e.g., additive manufacturing, 3D weaving) and are presented in the context of material architecture design for optimized mechanical, thermal, and/or fluid permeability properties.

Research on architected materials is growing rapidly due to the possibility to create materials with exotic properties, such as negative Poisson&’s ratio, negative refraction or extremely low mass density. These unusual properties arise from the geometry and structure of the material, rather than its bulk composition. Today&’s architected materials can be classified into two types: homogeneous (with periodic structures) and heterogeneous (with non-periodic structures). Most studies on architected materials focus on periodic arrangements of structures, and state of the art microstructural optimization is used to achieve desired material properties. However, designing complex structural assemblies to obtain spatially varying properties is much more challenging.
In our work, we present a complete framework for the fabrication of deformable objects with spatially tailored mechanical properties. First, we precompute a database of structures indexed by their mechanical properties, e.g., elastic modulus or Poisson&’s ratio. To achieve a large coverage in the mechanical properties space, we introduce a numerical optimization method for sampling structures with desired responses. Second, we synthesize the interior microstructure of a specific object by tiling microstructures drawn from the precomputed families. In a final step, we evaluate the resulting structure using a fully integrated FE analysis and compare results with the mechanical behavior obtained by quasi-static compression and tension tests. For the experimental testing we fabricate macro-scale samples using selective laser sintering as well as micro-scale samples using two-photon lithography. Our results indicate a good fit between the predicted and the measured mechanical properties. Furthermore, we show the possibility to change the non-linear behavior of the base material to a linear behavior by choosing a proper set of microstructure tiling geometry.

Along with their composition and atomic structure, the micro-architecture of biological materials strongly influences their mechanical properties. Nacre and bones are fascinating examples wherein intricate design motifs are employed to achieve outstanding strength. Nacre has high toughness as a result of its lamellar brick and mortar structure. Compact bone has a hierarchical assembly of lamellar tubes with varying orientation angle, leading to high fracture toughness. Toughening mechanisms arise at the nanoscale from the sliding of collagen fibrils and the formation and breakage of sacrificial bonds. In particular, interfacial friction plays a primary structural role in load transmission.
We use interleaved assemblies of paper sheets as a macroscale model system to study the contact mechanics and the origin of toughness in lamellar materials. We create a joint by interleaving the pages of a pair of identical paperback books (inspired by the “phonebooks problem” as seen in the TV show ‘MythBusters&’), and measure their strength and toughness by tensile testing. Through modeling and experiments, we identify how these properties scale with the architectural parameters of the joint: namely the number of sheets, the sheet thickness, the overlap among pages and the free lengths. Large joint strengths (> MPa) are obtained when the free length (defined as the length between the testing grip and the interleaved joint) is decreased and hence the page angles become larger.
The strength and toughness of these reversible, self-locking joints are highly dependent on the contact area among the overlapping pages. This seemed initially counter intuitive because friction force is known to be independent of contact area. In the regime of intermediate overlapping area, the peak tensile stress initially increases to 500 kPa, followed by a linear decline similar to strain softening, and exhibiting oscillatory stick-slip. We explain this in light of the micro-mechanics of friction by defining pressure dependent interfacial shear strength between 2 pages in contact. At large normal loads, micro-scale contact asperities between 2 pages flow plastically until they self-conform, the junction area becomes effectively equal to the apparent area, and the friction force saturates. This effect leads to high apparent plasticity during tensile loading, and eventual separation of the joint without fracture of the pages. In the large contact area regime, the books are inseparable and the pages start breaking in an effectively brittle regime. In sum, we show that, without the use of adhesives, the effective strength and ductility of laminated materials can be tuned simply by the architecture of interleaving and the micromechanics of friction. The insights from this study suggest the possibility for topological design of materials with unique mechanical properties arising from interlayer friction, and encourage investigation of these effects at small length scales.

An architecture comprised of many layers of hierarchical features had contributed to the overall stability of all life forms and objects. The smallest structural length-scale and largest length-scale of a material often differ by more than 5 orders of magnitude. Failure to fill this gap often results in a material with largely coupled mechanical properties, such as strength, ductility and density. For example, ultralight materials with volume fractions reduced to <1% than their parent material, have significantly degraded compressive and tensile strength 10^-4% of their parent material. Here we present hierarchical metallic materials with three dimensional features nested across over five orders of magnitude in length-scale, with configurable deformation modes separated by at least one order of magnitude at each level of hierarchy. We show that at very low relative densities the optimized materials with fractal hierarchy exhibit near linear scaling between strength-to-density. Remarkably, these meta-materials, with bend/stretch dominated hierarchical combinations, exhibit an elastomeric like, super elasticity that is super compressible and stretchable, with high specific strength and tensile strain greater than 20%.

The aim of this research is to introduce an analytical method that can rapidly calculate the dynamic response of a general multi-phased microarchitectured material (i.e., how the material&’s lattice deforms over time) when it is subjected to an ambient thermal load. A number of tunable three-phase microarchitectured material lattices that consist of compliant beam-based elements made of two different homogenous materials surrounded by void space have been designed such that the lattice can be made to achieve a desired bulk thermal expansion coefficient. Although analytical tools have been used to characterize and optimize the steady state deformation of these material lattices such that they achieve their final desired expansion when subjected to a change in ambient temperature, none of these tools consider the rate at which the different-material beam elements absorb the heat or the inertial effect of the mass within the lattice. Such considerations are critical to incorporate within the characterization and optimization tools for designing such microarchitectured materials because the material may not satisfy its intended functional requirements if its lattice contracts or expands in undesired ways before it reaches the final deformed state or if it rings with undesired vibrations induced by the thermal load. Whereas the final steady state deformation of traditional periodic microarchitectured material lattices can be calculated from the thermal response of a single repeated unit cell within the lattice, considerations of heat absorption and lattice inertia require characterizing the thermal response of every unit cell within the entire lattice simultaneously. Thus, an analytical method that can be used to rapidly characterize and optimize such lattices while considering these important time-dependent effects is not only necessary to the field, but such a method is significantly more complex and difficult to create. This paper introduces such a method and verifies is predictions via finite element analysis. Such a method could not only be used to characterize and optimize lattice designs that achieve tunable thermal expansion coefficients, it could also be used to design lattices that utilize internal dynamic vibrations to create desired bulk damping properties.

The tessellation of a large number of unit cells comprising slender beams leads to highly regular cellular materials, which can be designed, for instance, to achieve micro-architectured lattice materials with specific mechanical properties. In this paper, recent progress in developing materials with densities below 1000 kg/m³ and large specific strength, i.e. ratio of strength to density, will be discussed. On the one hand, it has been demonstrated that metallic and ceramic structures based on hollow beams with mass densities as low as about 1 kg/m³ can be realized. On the other hand, the mechanical properties, in particular the strength of such lattice materials is governed by the volume fraction of the constituent material. Also, the nodal connectivity of the truss structure has a pronounced influence.
In our work, we have designed specific micro-architectures, which are fabricated by applying 3D direct laser writing [1]. The structures consist of polymeric trusses with typical diameters in the range of 1 µm. The structures are coated with ceramic films by atomic layer deposition with thicknesses between 10 and 100 nm. Nanomechanical tests, employed to specifically designed test structures, have shown that this approach takes advantage of mechanical size effects for both the polymer and the ceramic films, as the strength of the materials at small scale exceeds the one of the bulk counterparts. With respect to the mechanical behavior of the truss structures, it will be shown that the specific strength can be further increased by specific heat treatments for improving the properties of the polymer as well as optimizing the ratio between thickness of the ceramic film to the diameter of the polymer truss.
It is also found that the elastic deformation behavior as well as the failure of the structure depends sensitively on details of the truss architecture and the loading conditions. For a detailed comparison between failure under compressive and tensile loading, push-to-pull structures were designed and tested. It turns out that certain truss designs show a pronounced asymmetry between tension and compression while others are less affected by the loading direction. Overall, it is found that tensile loading leads to lower strengths and brittle failure, presumably initiated by crack formation at stress concentrations at the nodes. Finally, the potential of further developments and some limitations in practical application of the materials are critically discussed.
[1] J. Bauer, S. Hengsbach, I. Tesari, R. Schwaiger, and O. Kraft “High-strength cellular ceramic composites with 3D microarchitecture” PNAS (2014)

Composites play an important role as structural materials in a broad range of fields due to their potential to combine beneficial properties of their constituents. In Nature, composites display fascinating architectures at multiple length scales, each of which can influence different properties. They are increasingly being used as an inspiration in developing novel materials. One particular principle of interest is the combination of hard building blocks, that constitute a majority phase, and a soft matrix phase, thus mimicking the microstructure of nacre, with its exceptional fracture toughness.
In this work we present a geometrical concept known as topological interlocking (Estrin et al., 2011), which can be utilised to vary the geometry of the hard building blocks and potentially produce structures with improved properties compared to a traditional brick-and-mortar design. The concept of topological interlocking is based on periodic assemblies of identical, discrete elements with specifically designed geometries, where each block is kinematically held in place by its neighbours. Our previous investigations have demonstrated that a ceramic plate segmented into interlocked elements can withstand flexural deflections ten times larger than those of a solid plate of the same thickness from the same material (Krause et al., 2012). It is anticipated that hybrid materials obtained by adding a soft phase to assemblies of topologically interlocked blocks will have superior mechanical properties.
With the aid of additive manufacturing, which has gained much attention due to its broad and far reaching potential applications, we can now readily access further design freedom which allows for the fabrication of complex geometries with fine features at micrometre resolution. Employing the state of the art 3D printing technology as a simple and efficient rapid manufacturing technique, we are able to print multiple polymeric materials with intricate inner architectures and widely contrasting mechanical properties within the same build. As a result, the ability to develop and investigate complex polymer composite assemblies, based upon the two design principles - topological interlocking and mimicking nacre - has become viable.
In this talk we will present results of a study detailing the various experimental and computational modelling work done to develop and fabricate a range of nacre-inspired, multi-material topologically interlocked structures using 3D-printing techniques.
References
Estrin, Y., A.V. Dyskin, and E. Pasternak (2011), Topological interlocking as a material design concept. Materials Science and Engineering C, 31, pp. 1189-1194.
Krause, T., Molotnikov, A., Carlesso, M., Rente, J., Rezwan, K., Estrin, Y., Koch, D. (2012), Mechanical Properties of Topologically Interlocked Structures with Elements Produced by Freeze Gelation of Ceramic Slurries, Advanced Engineering Materials 14, pp. 335-341.

Inverse opals are most widely used as photonic crystals for ultraviolet, optical and infrared applications.These highly interconnected porous structures are also attractive for applications such as sensors, fuel cells, filters, and catalysts. At the same time, engineers are aiming for lightweight structures with optimized mechanical strength, often inspired by nature&’s cellular materials with foam-like structures such as sponges, trabecular bone or plant parenchyma. The resultant optimized strut-based structures have shown high stiffness- and compressive strength-to-weight ratios, but can suffer from strut buckling and a lack of mass production techniques. Here we show that mechanical metamaterials based on ceramic inverse opaline structures with densities in the range of 330-910 kg/m³ are not only suitable as photonic crystals but also show better stiffness- and compressive strength-to-weight ratios compared to micro-fabricated optimized strut-based structures, but lower than carbon nanoframes fabricated by interference lithography. Pure silica inverse opal structures and silica inverse opals whose pores have been internally coated by a thin TiO₂-layer have been fabricated and their structural and mechanical behaviour was investigated. Our experimental results, supported by numerical simulations, show that these arch-shaped porous structures can outperform both strut-based and honeycomb structures due to their nearly isotropic mechanical response.

New weight efficient materials are needed to enhance the performance of vehicle systems allowing increased speed, maneuverability and fuel economy. Additive manufacturing combined with electroforming will be used to generate buckling optimized micro truss structures. A multi length-scale hybrid approach is applied to guide the optimization of the additive manufactured materials. The goal of the research was to expand material space for strength versus density allowing for reduced vehicle weight while maintaining strength. Fused Deposition Modeling (FDM) was used to take advantage of manufacturing flexibility, and electrodepositing was used to generate a high specific strength, bio-inspired hybrid material. The nano through meso scale characteristics/behaviors are analyzed with techniques ranging from EBSD to full scale composite panel testing to validate the optimized hybrid cores.
Microtension samples (3mm x 1mm with a 250µm x 250µm gage) were used to investigate the electrodeposited coatings in both the transverse (TD) and growth (GD) directions. Three bath chemistries were tested: copper, traditional nickel sulfamate (TNS) nickel, and nickel deposited with a platinum anode (NDPA). NDPA shows tensile strength upwards of 1600 MPa, significantly beyond the expected values reported in literature. This strengthening was linked to grain size refinement into the sub-30nm range, in addition to grain texture refinement resulting in only 17% of the slip systems for nickel being active. Anisotropy was observed in both nickel deposits which was linked to texture evolution inside of the coating. Information gained from microsample testing guided the selection of 15µm layer of copper deposition followed by a bulk layer of platinum deposited nickel.
Classical formulas for structural collapse were used to guide a experimental parametric study to establish a weight/volume efficient strut topology. Length, diameter and thickness were all investigated to determine the optimal column topology. It was discovered that the most optimal topology exists when Eulerian buckling, shell micro buckling and yielding failure modes all converge on a single geometric topology.
Three macro scale sandwich topologies (pyramidal, tetrahedral, and strut reinforced tetrahedral (SRT) were investigated with respect to strength per unit weight. The topologies were optimized across length scales using, texture on the nano-scale microsamples on the micro-scale and the parametric column study on the meso scale. The results validated additive manufacturing as a viable method for removing geometric constraints observed by other manufacturing methods. The SRT was the most optimized topology showing the highest strength per unit weight. The final topology sits in a best-of-both area of material space exceeding the commercially available honeycombs strength per relative density by 1670% and strength per weight by 507%.

Recent advances in direct manufacturing open the door for promising materials such as ordered structural foams. Reduced bending in closed cell stochastic foams, compared to open cell, gives rise to substantially higher stiffness and strength. Closed cell ordered foams similarly outperform lattice materials and can achieve theoretical bounds for stiffness (Hashin and Shtrikman, 1963). Mechanical models for the stiffness of foams (Gibson, 1989 & Grenestedt, 1999) and theoretical bounds are used as metrics to compare a variety of ordered foam topologies. Representative volume element (RVE) finite element modeling (FEM) is used to calculate strain energy distributions to identify features common in high performance designs. Three classes of materials are identified: maximal performance designs with a total stiffness that approaches theoretical bounds at low relative densities, a high performance stretch dominated group, and a compliant group with high mesoscale configurational entropy. A variety of historical and novel topologies are considered, including open cell designs, that largely represent the performance of stiff mesoscale ordered materials, eliciting a wide range of isotropy, over the range of low to intermediate relative densities. One design in particular achieves theoretical bounds for isotropic stiffness in the low density limit and maintains greater than 96% of the theoretical upper bounds though 40% relative density. This material is composed of two highly anisotropic substructures whose relative density can be scaled independently allowing the isotropy to be tailored. In this light, this material is amenable to optimization to produce functionally graded multicellular designs. Test specimens consisting of single unit cells were fabricated using a Stratasys uPrint SEtrade; Plus 3-D printer out of an ABS material (ABSplustrade; - P430). Approximately 100 ~0.5mm thick layers composed each specimen. A sacrificial support material is removed using a lye bath and agitator. Compression experiments were conducted on four specimens using a servo-hydraulic press both parallel (n=3) and perpendicular (n=1) to the print direction. Experiments agree quantitatively with finite element (FE) models exhibiting peak strengths that are ~75% of Voigt bound. This suggests, assuming no strain hardening in the ABS plastic, ~75% of the material has yielded at peak load. No significant difference in stiffness or peak strength was observed between compression parallel and perpendicular to the print direction however delamination of printed layers was prominent in the latter. Single unit cell structures and the corresponding periodic material are predicted to behave similarly in FE models suggesting little edge effects. Attempts are being made to fabricate test specimens out of 304 steel/Brass composite and TiC. The TiC material has the potential to outperform existing materials by nearly an order of magnitude in terms of specific stiffness.

A continuum model is developed for hexagonal lattices, composed of a set of masses connected by linear axial and torsional springs, with nonlinearity arising solely from geometric effects. For a set of lattice parameters, these lattices exhibit complex deformation patterns under uniform loading conditions due to instabilities. A continuum model accounting for these instabilities is developed from explicit expressions for the potential energy functional of the lattice unit cell. This functional is non-convex, captures the bistable nature of the lattice and is used to derive a constitutive model for the lattice. Finite element analysis of continuum media illustrate the formation of microstructure patterns with discontinuous displacement gradients, similar to the features observed in nonlinear elasticity and finite deformation plasticity. A comparison of discrete lattice simulations and finite element analysis under general loading conditions is presented, illustrating that our continuum model captures the effective behavior due to instabilities arising in the lattice. Furthermore, the developed model allows the explicit computation of tangent stiffnesses, which are utilized to investigate the wave propagation characteristics of the lattices under different states of applied stress, and in particular near the onset of instabilities. Of interest, is the extension of the pattern-forming predictions to the dynamic regime, which is relevant to wave guiding and load path design under high amplitude mechanical loads.

Artificially structured composite materials that enable manipulation and control of sound waves in air have received significant interest in recent years, not only because of their rich physics, but also for their broad range of applications. Here, we report a new class of tunable and switchable acoustic metamaterials composed of three-dimensional stretchable chiral helices in air arranged on a two-dimensional square lattice. Through a combination of Finite Element analyses and desktop-scale experiments we demonstrate that the deformation of the helices can be exploited to control the propagation of pressure waves in the surrounding air. In fact, our results indicate that the bandgap of the acoustic metamaterial can be tuned and even suppressed by increasing the amount of applied deformation, paving the way to the design of a new class of acoustic switches with on/off capabilities.

From semiconductors to metallurgy, defects play a key role in material properties. A direct consequence of defects in a lattice is the existence of localised states of vibration. These defect modes have been studied from multiple angles, but the tuning of a material's properties through the excitation of defect states remains unexplored. In this work, we show that the excitation of a defect mode can result in extreme mechanical properties, such as negative or infinite stiffness. In addition, the excitation parameters, the frequency and the amplitude, provide a high degree of control on the mechanical response. This allows to selectively tune the incremental stiffness at arbitrary points in a material's force-displacement relation, a feature that does not exist in other systems with tunable stiffness. The stiffness tunability originates in the nonlinear coupling between the oscillation of the defect mode, the force at the boundary of the material, and the deformation. This talk will provide a description of the stiffness tuning mechanism, as well as numerical and experimental demonstrations of this phenomenon in a one-dimensional lattice of steel spheres.

In recent years, advances in three-dimensional lithographic techniques have allowed for the fabrication of architected materials with precisely controlled topology and micro- to nanoscale periodicity. Many such architected materials possess photonic properties and photonic bandgaps within particular wavelength ranges, making them photonic crystals. Broadly tunable photonic crystals in the near- to mid-infrared region could find use in spectroscopy, non-invasive medical diagnosis, chemical and biological sensing, and military applications, but so far have not been widely realized. We report the fabrication and characterization of three-dimensional (3D) tunable photonic crystals composed of polymer nanolattices with an octahedron unit-cell geometry. These photonic crystals exhibit a strong peak in reflection in the mid-infrared that shifts substantially and reversibly with application of compressive uniaxial strain. A strain of ~40% results in a 2.2mu;m wavelength shift in the pseudo-stop band, from 7.3mu;m for the as-fabricated nanolattice to 5.1mu;m when strained. We found a linear relationship between the overall compressive strain in the photonic crystal and the resulting stopband shift, with a ~50nm blueshift in the reflection peak position per percent increase in strain. These results suggest that architected nanolattices can serve as efficient three-dimensional mechanically-tunable photonic crystals, providing a foundation for new opto-mechanical components and devices across infrared and possibly visible frequencies.

Thermoelectric material research is driven by the goal to minimize the thermal transport, which yields high thermoelectric figure of merit. In semiconductor materials heat is mainly carried by high frequency phonons. As the propagation of phonons in atomic lattices bares similarities to the propagation of elastic waves in microscale systems, the atomic structure can be used as a design guideline for novel microlattice materials.
For instance, phononic superlattices are commonly used to alter phonon propagation. The basic principle involes the stacking of layers with a mismatch in their characteristic impedance. If the wavelength of incoming elastic-waves, or phonons, is in the order of the stacking period, interference between the incoming and reflected waves occurs and yields a band gap in the frequency spectrum. In this work we exploit the analogy between phonons in atomic superlattices and the propagation ultrasound in micro scale superlattice, to create a novel material for ultrasound control. We show the occurence of band gaps that can be tailored to a specific frequency between 1 and 20 MHz. We use the specific lattice geometry of Si/Ge superlattices to guide the design of a micro phononic superlattice. Moreover we employ a 2PP 3D-Lithography method to create fully 3D polymeric lattice geometries with minimum feature sizes of less than 10 mu;m. The overall mechanical properties are controlled by means of varying the independent trusses that constitute the unit cell. We use a numerical model including fluid structure interaction to predict the occurrence of band gaps. Finally, the numerical model is validated using an experimental setup with high frequency ultrasonic transducers.

Topology optimization is the most power tool for designing materials with unprecedented topological complexity, such as multiphase cellular materials. This technique allows for efficient development of feasible features within a multiphase design domain, potentially leading to unique material properties. Although stiffness optimization of lightweight cellular materials and structures made of a single constituent phase has been extensively investigated, the application of topology optimization to more complex objective functions and multiphase cellular material systems is still in its infancy. In this presentation, we will discuss a systematic optimal design approach for multiphase periodic hybrid cellular materials with ideal combinations of high stiffness, low density, and high loss coefficient, resulting in maximum vibration damping under wave propagation. In the suggested framework for the multiphase microstructure, each point within the design domain can take a combination of phases and we adopt a penalization approach to achieve a clear phase separation. We utilize classic homogenization theory to model the effective stiffness of the unit cell and the Bloch-Floquet approach to obtain the damping capacity of the microstructure. The effects of wave frequency, damping properties of each individual phase, and existence of each phase on the optimal design will be discussed. The proposed approach can optimize the damping performance for a range of frequencies, as opposed to single frequency, thus leading to more robust designs.

Acoustic metamaterials utilizing local resonances are known to produce low frequency band gaps that break the limit of typical Bragg scattering mechanisms, however these band gaps tend to be quite narrow. We present on a new type of acoustic meta-structure that can simultaneously produce both low frequency and wide band gaps, by coupling a truss lattice with periodic local resonators to induce different local resonator modes. Analytical models that reduce the mode shapes to a 1D approximation inform the lattice geometry design. Specifically, by analyzing beam deformations of the individual truss elements, we can modify simple geometries to create low-stiffness geometries. Numerical modeling results show the ability to control band gap width and center frequency by varying the geometry, resonator material, resonator volume fraction/spacing, and truss dimensions. By using novel 3D printing techniques, we can fabricate functional composite polymeric-metallic structures, as well as easily fabricate complex geometries. The presence of low and wide band gaps is shown through finite element modeling and transmission experiments with laser vibrometers on 3D printed meta-structures. We show tunability of the band gaps by optimizing the lattice geometry. These acoustic meta-structures are designed for applications in structural vibration mitigation and sound absorption, but manufacturing on different size scales could open up applications in higher frequency ranges, e.g. for frequency filtering in MEMS devices.

Phononic crystals are periodic elastic structures which exhibit a range in frequency where elastic wave propagation is barred. The ability to design structures with such phononic bandgaps has been of growing interest in recent years due to their potential as sound filters, acoustic mirrors, acoustic wave guides, and vibration solators and in transducer design. Typical structures take the form of 2D or 3D arrays of inclusions of one or more materials in a matrix with contrasting properties. The position and width of the gap can be tailored by the selection of (a) constituent materials with contrasting densities and contrasting speeds of sound, (b) lattice topology and (c) volume fraction of inclusions. Many solid-air structures have been pursued through experiments and/or simulations , but most of them are characterized by a high porosity ( > 40%), limiting their potential applications in many scenarios. Here, we introduce a novel design for 2D phononic crystals comprising arrays of mutually orthogonal elongated voids. Importantly, our study shows that in such structures a bangap exists even for porosities as low as 5%. We also show that the smallest structure ligament size controls the appearance and evolution of the bandgaps and experimentally verify our findings.

One of the primary vibroacoustic challenges faced by sandwich panel designers is reducing the acoustic power transmitted through these structures without adding significant mass or compromising stiffness. In this study, sandwich beams with a variety of truss cores are investigated experimentally and theoretically to understand the structure-property relationships between core architecture and vibroacoustic performance. Sandwich beams are fabricated with HRL&’s polymer truss, formed from an interpenetrating array of self-propagating waveguides cured under ultraviolet light. The speed of transverse waves is measured through each beam as a function of excitation frequency and the wave speeds are used to determine the coincidence frequency of each beam—a key to understanding acoustic transmission through the structure. Quasi-static measurements are performed to determine the homogenized elastic and shear modulus of the truss structure and these values are incorporated into a previously-developed consistent higher-order model used to describe sandwich panel vibration. Symmetric and antisymmetric facesheet motion is mathematically decoupled, and the relation between excitation frequency and wavenumber in antisymmetric facesheet motion is solved impli