Behavior Of Metamaterials Research Articles

Non-linear mechanics of mechanical metamaterials

To elucidate the static and dynamic behaviors of mechanical metamaterials, we mechanistically constructed a metamaterial structure using bars, discs, springs, and bolts, and conducted in-situ tensile experiments. We found that the system undergoes multiple structural transformations by an imposed strain. We inferred that the multi-stability and bifurcation play an important role in non-linear mechanics of mechanical metamaterials.

A note on the multiscale volume changes of auxetic metamaterials assessed via digital image correlation

The present manuscript approaches the mechanical interpretation about the volume changes of auxetic metamaterials within a multiscale perspective. The kinematic fields resulting from the mechanical tests were assessed via digital image correlation (DIC). The multiscale mechanical characterization is grounded within sound homogenization principles based on representative volume elements (RVE) at large deformations. The main achieved results concern in highlight that even though the Poisson’s ratio still widely used to characterize the mechanical behavior of auxetic metamaterials, it does not represent a suitable variable to quantify the multiscale volume changes, mainly when finite deformations take place. In this regard, a proper homogenized kinematical quantity is used in this work. Moreover, statistical tools are employed aiming at a better understanding to what extent the multiscale kinematics influence the volume changes and the overall auxetic behavior of the material.

Brochosome-Inspired Metal-Containing Particles as Biomimetic Building Blocks for Nanoplasmonics: Conceptual Generalizations.

Recently, biological nanostructures became an important source of inspiration for plasmonics, with many described implementations and proposed applications. Among them are brochosome-inspired plasmonic microstructures—roughly spherical core-shell particles with submicrometer diameters and with indented surfaces. Our intention was to start from the nanoplasmonic point of view and to systematically classify possible alternative forms of brochosome-inspired metal-containing particles producible by the state-of-the-art nanofabrication. A wealth of novel structures arises from this systematization of bioinspired metal-containing nanocomposites. Besides various surface nanoapertures, we consider structures closely related to them in electromagnetic sense like surface nano-protrusions, shell reliefs obtained by nano-sculpting, and various combinations of these. This approach helped us build a new design toolbox for brochosome-inspired structures. Additionally, we used the finite elements method to simulate the optical properties of simple brochosome-inspired structures. We encountered a plethora of advantageous optical traits, including enhanced absorption, antireflective properties, and metamaterial behavior (effective refractive index close to zero or negative). We conclude that the presented approach offers a wealth of traits useful for practical applications. The described research represents our attempt to outline a possible roadmap for further development of bioinspired nanoplasmonic particles and to offer a source of ideas and directions for future research.

Mechanical Metamaterials with Discontinuous and Tension/Compression‐Dependent Positive/Negative Poisson's Ratio

Ordinary materials have positive Poisson's ratios (PRs), and the currently known mechanical metamaterials possess negative PRs both under tension and compression. Herein, two types of novel cellular structure units with tension/compression‐dependent positive/negative PRs are demonstrated. For type I structure, the unit exhibits a positive PR during tension and a negative PR during compression, while for type II structure, the unit shows a negative PR during tension and a positive PR during compression. The deformation behaviors of the new metamaterials are first predicted by a simple analytical model and finite‐element method calculations, and the predicted results are then confirmed experimentally. Numerical and experimental studies show that for the plane angle , the unit of the new metamaterial switches its PR under compression and exhibits a discontinuous change in the PR. The deformation law does not depend on either the Young's modulus or the hardness of the material. The geometrically determined deformation behaviors of the proposed metamaterials are observed in 2D and 3D cellular structures and are proven to be robust through numerical and experimental studies. The proposed mechanical metamaterials can be applied as new deformation‐protected materials in energy‐absorbing devices, soft robotics, and tissue engineering scaffolds and as thermal deformation materials.

Out-of-plane deformation reduction via inelastic hinges in fibrous metamaterials and simplified damage approach

The mechanical behavior of fibrous metamaterials is mainly determined by the interactions between the fibers composing the architecture. These interactions are usually of two different kinds: those directly depending on the positions of the fibers and those that need mediators, usually consisting of hinges, either inelastic or perfect, inducing restrictions on the kinematics of the fiber joints. In cases of interest, it has been observed that hinges can either have a certain torsional stiffness or behave as perfect joints, simply ensuring that the fibers remain interconnected, but not applying any constraint on the relative rotations between them. Here the effect of torsional stiffness of inelastic hinges is studied in two shear tests for a selected fibrous metamaterial. It is shown that the stiffness of hinges can be tailored to avoid, or at least reduce, out-of-plane deformations. Moreover, it is shown that, after reaching a threshold, permanent deformations are observed. This phenomenon is treated in a simplified way, by introducing damage in the continuum model.

Vat photopolymerization of fly-like, complex micro-architectures with dissolvable supports

Recent advances in additive manufacturing of complex geometries enabled the creation of mechanical metamaterials whose exotic properties are based on local control of complex cell geometries. Overhanging and free-hanging features that lack continuous support layers in the previous build volume cannot be directly manufactured, imposing a major design limitation. The resulting metamaterials are limited to single homogenous structural materials, and inherently self-supporting geometries, resulting in constraints of achievable architectures. Realizing arbitrary features is compelling but is inherently limited by process and material support constraints. Here we present a novel light-based additive manufacturing approach capable of printing arbitrary micro-architectures comprising a large array of internally suspended features, large span overhang, and high aspect ratio struts. This method eliminates the need for manual removal of internal supports and enables a suite of multi-functional metamaterials with a range of designed properties, including wide bandgaps for elastic waves at low frequency, switchable wave transmissions, and products requiring no post support removal. We describe the synthesis and rapid printing of a variety of metamaterials comprising an extensive array of suspended features and demonstrate their metamaterial behaviors. The proposed approach removes scale and unit cell limitations and is capable of achieving embedded features across multiple materials.

Ultrabroadband electrically controllable terahertz modulation based on GaAs Schottky diode structure

We demonstrate an ultrabroadband electrically controllable terahertz modulator based on a Schottky diode structure formed with periodic metal microslits on an n-doped GaAs substrate. The mechanism of our design is different from that of the traditional Schottky diode-based THz electrical modulator, which uses free charge carriers in a substrate to control the resonant behavior of metamaterials. In our device, the modulation is based on free-carrier absorption on the THz wave and therefore broadband. The charge carrier concentration between the metal microslits is actively modified by applying a reverse bias voltage to generate a direct modulation of THz waves. The modulation performance is enhanced by the THz non-resonant electric field enhancement effect from the metal microslits. The experimental results indicate that the modulation depth is positively correlated with the electric field enhancement ratio at the depletion region in the gap and the number of microslits in the THz light spot-covered area. An averaged modulation depth of ∼40% in the measurable frequency range from 0.4 to 1.4 THz was achieved by the device with a metal microslits gap width of 2 µm and a period of 20 µm. A maximum modulation depth of ∼75% was achieved by stacking two devices back-to-back with a 3-dB down bandwidth modulation speed of ∼100 kHz. Further improvements of the device can be achieved by optimizing the parameters such as the free-carrier density in the doping layer, the active area size, and the specifications of the metal microslits.

Geometrically nonlinear frequency analysis of composite cylinders with metamaterial honeycomb layer and adjustable Poisson’s ratio using the multiple scale method

This article presents an analytical method to study the nonlinear frequency of composite cylindrical shells with metamaterial honeycomb core layer and adjustable Poisson’s ratio. The governing equations for the axisymmetric case, which are coupled nonlinear partial differential equations, are obtained based on the Mirsky–Hermantheory and the von-Karman nonlinear relations. These equations are solved analytically using the multiple scale method and the linear and nonlinear frequencies are determined. By conducting a parametric study, the effects of different mechanical and geometrical parameters are investigated on composite shells. It is observed that by changing the geometrical parameters of the honeycomb layer, a vast domain of the Poisson ratios from negative, zero, and positive values are accessible to achieve a composite structure with metamaterial behavior. Since the linear natural frequency, the coefficient of nonlinear frequency, and the weight of the shell will be changed by variations of the Poisson ratio, it will give us this opportunity to adjust them to a suitable value by changing the geometrical parameters of the honeycomb structure. To study the accuracy of the presented method, the results are compared with some other references and the finite element analysis.

A Stair‐Building Strategy for Tailoring Mechanical Behavior of Re‐Customizable Metamaterials (Adv. Funct. Mater. 37/2021)

Re-Customizable Metamaterials In article number 2101808, Fei Pan, Yuli Chen, and co-workers develop a “stair-building” strategy for customizing and re-customizing targeted mechanical behaviors for mechanical metamaterials. Similar to building a stair with bricks, a targeted stress-strain curve can be realized by stacking the brick-shaped loading curves of bistable units visually. Different stress-strain curves can be re-customized in a large range using the same piece of metamaterial. This strategy provides a new perspective for rapidly tailoring the mechanical behavior of re-customizable metamaterials.

A dual-band, dual-polarized, CPW-fed wideband antenna loaded with via less CRLH-MTM TL for 5G mm-Wave communication

This paper effectively implements the process of bandwidth enhancement and realization of circular polarization (CP) of metamaterial-based antenna by loading composite right/left-handed (CRLH) transmission line (TL) unit cell without vias. This antenna is a symmetrical structure that comprises of square slot, two asymmetric meander lines (AML) in y-direction, an interdigital capacitance (IDC) in x-direction, inductive rhombus (IR), and coplanar waveguide (CPW) feed line in a single plane. Mender lines (ML) provides the needed shunt inductance whereas IDC in the signal line provides series capacitance for left-handed TL. The proposed metamaterial-based antenna structure essentially produces three resonating bands. The simulated first band has a frequency range of 14.5 GHz to 15.4 GHz due to zeroth-order resonance (ZOR), which can be used for satellite communication. Dispersion analysis is done to verify metamaterial (MTM) behavior at ZOR frequency using Bloch-Floquet theory. The second and third resonance are due to coupling capacitances, created in antenna structure, which are merged to make a broadband antenna in millimeter-Wave (mm-Wave) range from 28 GHz to 39.2 GHz, for 5G cellular communication by increasing the diagonal length of IR. The CP is attained by converting symmetric meander lines (MLs) to asymmetric MLs to excite the orthogonal field components of equal amplitude. The simulated results are compared to measured results and found to be in a good match. The measured impedance bandwidth (IBW) for the Ku-band is 7.31% (14.5–15.6) GHz with linear polarization (LP) characteristics while 31% (28–38.2) GHz in mm-Wave (Ka-band) range. The measured axial ratio bandwidth (ARBW) is 5.2 GHz (13.9%, 33.6 GHz-38.8 GHz) in mm-Wave range. The proposed antenna has a peak gain of 6.7 dBi and maximum efficiency is 93%. The overall dimension of the presented antenna is 0.5λ0 × 0.5λ0 × 0.025λ0 at 14.9 GHz.

A 3D Mechanism-driven Hexagonal Metamaterial: Evaluation of Auxetic Behavior

This paper investigates an unconventional 3D printable auxetic metamaterial realized by a unique sliding mechanism. The concept of 3D sliding induced hexagonal auxetic (SIHA) metamaterial is proposed with stable auxetic behavior and improved compression load response, as compared with the conventional re-entrant structure. For a single SIHA lattice cell, the theoretical model concerning the sliding mechanism is developed and verified by both finite element analysis (FEA) and experiment. Furthermore, the compression properties of the periodic SIHA structure are evaluated by FEA with experimental verification. A 3D conventional re-entrant honeycomb (REH) structure is adopted for comparison. It is shown that the FEA results agree with the experimental results, and overall superior performance of SIHA structure is obtained, which is reflected by higher compression resistance and more stable auxetic behavior. Moreover, the sensitivity of performance with respect to the friction coefficient in the sliding mechanism is investigated. It is found that the performance and auxetic behavior of the SIHA metamaterial is sensitive to the friction condition. Force-strain curve increases with the input friction coefficient, while an excessive friction coefficient may cause undesirable effects on the slip mechanism. Overall, the friction sliding and collapsing structural mechanism-driven structures could provide a new way to entrap elastic energy under compression deformations.

Compression-torsion conversion behavior of a cylindrical mechanical metamaterial based on askew re-entrant cells

The compression-torsion conversion (CTC) behavior of a novel cylindrical mechanical metamaterial is studied in the present paper. The structure is composed of arrayed askew re-entrant cells that originate from the traditional re-entrant cell. The effects of geometrical parameters such as the shape of askew re-entrant cells, the cell number in each layer and the radial thickness of the structure on the CTC efficiency are investigated via finite-element simulations and experiments. The numerical results indicate that the shape of the askew re-entrant cells mainly determines the CTC efficiency. Moreover, the proposed structures are fabricated utilizing additive manufacturing technology. Research results show that the cylindrical mechanical metamaterial features excellent CTC property.

Nonlinear multiscale simulation of elastic beam lattices with anisotropic homogenized constitutive models based on artificial neural networks

A sequential nonlinear multiscale method for the simulation of elastic metamaterials subject to large deformations and instabilities is proposed. For the finite strain homogenization of cubic beam lattice unit cells, a stochastic perturbation approach is applied to induce buckling. Then, three variants of anisotropic effective constitutive models built upon artificial neural networks are trained on the homogenization data and investigated: one is hyperelastic and fulfills the material symmetry conditions by construction, while the other two are hyperelastic and elastic, respectively, and approximate the material symmetry through data augmentation based on strain energy densities and stresses. Finally, macroscopic nonlinear finite element simulations are conducted and compared to fully resolved simulations of a lattice structure. The good agreement between both approaches in tension and compression scenarios shows that the sequential multiscale approach based on anisotropic constitutive models can accurately reproduce the highly nonlinear behavior of buckling-driven 3D metamaterials at lesser computational effort.

Negative Index Metamaterial Lens for Subwavelength Microwave Detection

Metamaterials are engineered periodic structures designed to have unique properties not encountered in naturally occurring materials. One such unusual property of metamaterials is the ability to exhibit negative refractive index over a prescribed range of frequencies. A lens made of negative refractive index metamaterials can achieve resolution beyond the diffraction limit. This paper presents the design of a metamaterial lens and its use in far-field microwave imaging for subwavelength defect detection in nondestructive evaluation (NDE). Theoretical formulation and numerical studies of the metamaterial lens design are presented followed by experimental demonstration and characterization of metamaterial behavior. Finally, a microwave homodyne receiver-based system is used in conjunction with the metamaterial lens to develop a far-field microwave NDE sensor system. A subwavelength focal spot of size 0.82λ was obtained. The system is shown to be sensitive to a defect of size 0.17λ × 0.06λ in a Teflon sample. Consecutive positions of the defect with a separation of 0.23λ was resolvable using the proposed system.

Holey-structured tungsten metamaterials for broadband ultrasonic sub-wavelength imaging in water.

Metamaterials exhibiting Fabry-Pérot resonances are shown to achieve ultrasonic imaging of a sub-wavelength aperture in water immersion across a broad bandwidth. Holey-structured metamaterials of different thickness were additively manufactured using a tungsten substrate and selective laser melting, tungsten being chosen so as to create a significant acoustic impedance mismatch with water. Both broadband metamaterial behavior and sub-wavelength imaging in water are demonstrated experimentally and validated with finite element simulations over the 200-300 kHz range.

An octagonal split ring resonator-based double negative metamaterial for S-, X- and Ku-band applications

This study aimed to produce a miniaturised double negative metamaterial with a lower resonance frequency. Therefore, a new combination of a multi-circular ring connected with octagonal shape ring metamaterial was developed on a 9 × 9 mm2 dielectric substrate material with a thickness of 1.6 mm, named the Flame Retardant 4 (FR-4). While the selected frequency ranged between 0 and 18 GHz for the unit and array metamaterial design. Numerical simulation was used for the design and analysis of the proposed metamaterial. A few analysis were performed to validate the performance of the new design including the analyses of the different dimensions of the substrate and varying widths of the split gaps. The proposed design structure manifested resonance frequencies at S, X and Ku-bands. The resonance frequency at S-band (3.31 GHz) and X-band (8.60 GHz) presented double-negative (DNG) metamaterial behaviour while X-band (11.93 GHz) and Ku-band (12.99 GHz) presented single-negative (SNG) medium characteristics. The simulation and measured results almost coincided with each other. The compactness of the proposed design was proven by the effective medium ratio (EMR) of 10.07. In conclusion, the miniaturised structure can be accredited for satellite communication and radar applications.

Modeling and Analysis of the Transient Behavior of an Elastic Metamaterial as a Generalized Cosserat Continuum

Abstract Metamaterials are man-made materials engineered to possess certain desired and often counterintuitive properties. It is well-known that elastic metamaterials may exhibit unusual bulk elastic properties when subject to dynamic loads at certain frequencies, such as negative Poisson’s ratio, negative modulus, and/or negative mass density. This paper focuses on the modeling of a one-dimensional micropolar-type elastic metamaterial subject to transient dynamic loading. The metamaterial consists of unit cells that support both translational and rotational motion and can be modeled as a micropolar-type continuum. Interestingly, the equations governing the dynamics of the new micropolar-type continuum possess remarkable similitude with those governing the transient response of an elastic bar with elastic supports. The resulting governing equation is solved using the Fourier transform technique. The transient dynamic response of the metamaterial subject to an axial impact is then studied based on both the developed continuum model and the original discrete model. Finite element analysis of the equivalent bar model is also conducted. The results from the developed continuum model is compared with the discrete model and the finite element analysis to evaluate its suitability. This work presents a novel micropolar-type model for a specific metamaterial and investigates its transient response using both continuum modeling and discrete unit cell modeling. The results indicate that the micropolar-type model can accurately capture the transient behavior of the propagation of the strain pulse in the metamaterial

Hydrogel-driven origami metamaterials for tunable swelling behavior

Origami metamaterial has gradually shown its potential in the fields of science and engineering due to its unique mechanical behaviors. Since hydrogel shows a positive volume change when absorbing water, it provides a new design strategy for actively deformed origami metamaterials composed of normal material framework and active hydrogel layer. In this paper, we proposed three synergistic mechanical designs with positive/negative/zero tunable swelling deformation by adjusting the opening direction of the circular holes. The deformation behaviors of the origami metamaterials manufactured by a multi-material 3D printer were qualitatively and quantitatively characterized and demonstrated, using finite element analysis, experimental test and theoretical method. The results indicate that the origami metamaterials present significant tunable swelling behaviors, which can be customized by adjusting the lattice microstructure geometry parameters. Utilizing the origami metamaterials, cylindrical shell and box structures with tunable swelling behavior were designed. Additionally, the hydrogel-driven actuator based on the origami metamaterials was manufactured to pull/drag an underwater object. Since the tunable swelling behaviors are not related to scale, these hydrogel-driven origami metamaterials can be applied to the design of reconfigurable and programmable devices on the macro and micro scale and possess promising potential for wide applications in bioelectronics and biological tissue engineering.

Nonlocal effective-medium theory for periodic multilayered metamaterials

Spatial-dispersion (nonlocal) effects are non-negligible in periodic multilayered metamaterials under certain specific conditions, which cannot be completely understood based on the local effective-medium theory, even though the metamaterials are constructed by deep subwavelength meta-atoms. Here, we present a simple yet robust effective-medium model for such media in which the nonlocal effects are properly considered. Our proposed nonlocal model is established by the analysis of the dispersion relation of the effective medium without any expansion-based approximation, which is applicable for description of the optical behavior of the multilayered metamaterials even under critical conditions, and works well for both TE and TM polarized waves. We believe our model will be a powerful tool for the investigation of electromagnetic nonlocality in the realm of metamaterials and subwavelength optics.

Polarization-independent multiband double-negative metamaterial through ferrite-based flexible substrate with tunable microwave dielectric properties

Microwave flexible dielectric materials offer a new set of attractive applications for modern devices as well as for metamaterial research and technology. In this article, metamaterials with double-negative characteristics fabricated on a novel flexible dielectric substrate are presented. These novel flexible microwave substrates were prepared from Mg-Zn ferrite powder. The proposed flexible metamaterials offer double-negative (DNG) properties, polarization insensitivity, and a higher effective medium ratio (EMR) for multiple bands of application. The prepared, flexible substrates exhibit performance superior to commercial materials with tunable dielectric properties, and the proposed metamaterial structure enhances the antenna performances as well. The flexible substrates are synthesized from Mg-Zn ferrite having two different molecular compositions of Mg denoted as Mg40 and Mg60. The prepared, highly flexible, and lightweight magnesium zinc ferrites of Mg40 and Mg60 possess tunable dielectric properties with relative dielectric permittivity of 5.18 and 4.20 and dielectric loss tangents of 0.004 and 0.006, respectively. The double-negative (DNG) metamaterial behavior is observed with the fabricated flexible metamaterials at 2.92 GHz, 4.42 GHz, 7.75 GHz, and 9.84 GHz for Mg40 and 3.16 GHz, 4.71 GHz, 8.21 GHz, and 10.52 GHz for Mg60 and validated by using 3D electromagnetic Simulator CST (Computer Simulation Technology) Microwave Studio and ADS (Advanced Design System) software. Therefore, the prepared, flexible metamaterials having improved EMR, polarization insensitivity, and DNG properties on a MgZnFe2O4substrate are suitable additions to the field of the flexible microwave.