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We make precise some results on the cloaking of displacement fields in linear elasticity. In the spirit of transformation media theory, the transformed governing equations in Cosserat and Willis frameworks are shown to be equivalent to certain high-contrast small defect problems for the usual Navier equations. We discuss near-cloaking for elasticity systems via a regularized transform and perform numerical experiments to illustrate our near-cloaking results. We also study the sharpness of the estimates from [H. Ammari, H. Kang, K. Kim, and H. Lee, J. Differential Equations, 254 (2013), pp. 4446-4464], wherein the convergence of the solutions to the transmission problems is investigated, when the Lamé parameters in the inclusion tend to extreme values. Both soft and hard inclusion limits are studied and we also touch upon the finite frequency case. Finally, we propose an approximate isotropic cloak algorithm for a symmetrized Cosserat cloak. [ABSTRACT FROM AUTHOR]

A broadband ultra-large area acoustic cloak based on fluid medium was designed and numerically implemented with homogeneous metamaterials according to the transformation acoustics. In the present work, fluid medium as the body of the inclusion could be tuned by changing the fluid to satisfy the variant acoustic parameters instead of redesign the whole cloak. The effective density and bulk modulus of the composite materials were designed to agree with the parameters calculated from the coordinate transformation methodology by using the effective medium theory. Numerical simulation results showed that the sound propagation and scattering signature could be controlled in the broadband ultra-large area acoustic invisibility cloak, and good cloaking performance has been achieved and physically realized with homogeneous materials. The broadband ultra-large area acoustic cloaking properties have demonstrated great potentials in the promotion of the practical applications of acoustic cloak. [ABSTRACT FROM AUTHOR]

The presence of extreme material properties and undesired scattering make practical implementation of transformation optics based cloaks an extremely challenging issue. To overcome these problems, a simplified quadratic cloak has been proposed which provides finite material properties at the inner radius of the cloak and impedance match with the free space at the outer radius of the cloak simultaneously. The simultaneous occurrence of finite material properties and impedance match with the free space at the respective boundaries reduces the scattering cross section significantly. The material properties of the proposed simplified quadratic cloak can be realized with two dimensional (2D) metamaterials. The performance of the proposed cloak is examined by plotting the normalized total scattering cross section (SCSt,norm), and the scattering patterns in azimuthal plane. The proposed simplified quadratic cloak shows 10 dB reduction of the scattered field in both the forward and backward directions with respect to the perfect electric conductor (PEC) cylinder. [ABSTRACT FROM AUTHOR]

• Topology optimization-based design of thermo-mechanical metadevices is proposed. • The so-designed devices can be manufactured by using simple isotropic materials. • The devices performance has been analyzed via finite element method simulations. • Such devices exhibit a good performance, when compared to more complex metadevices. The present work has been conducted in order to introduce a novel approach for the design of mechanical devices conceived to manipulate the displacements field in linear elastic materials subjected to thermal gradients. Such an approach involves the solution of a topology optimization problem where the objective function defines the error in achieving a prescribed displacement field, and the mechanical device consists of two macroscopically distinguishable isotropic candidate materials. The material distribution is defined as a continuous function by following the solid isotropic microstructure (or material) with penalization (SIMP) method. The so-designed devices are easy to manufacture, since the design variables dictate the candidate materials distribution. Based on such an approach it is not necessary to devise further ways to simultaneously mimicking several thermal and mechanical effective properties, as required by coordinates transformation-based metamaterial design methods. Although the candidate materials are isotropic, the mechanical device behaves as a metamaterial allowing the desired manipulation of the displacements field. As an example, this topology optimization-based approach is applied to the design of an elastostatic cloaking device subjected to thermal gradients, considering also thermo-dependent mechanical properties. [ABSTRACT FROM AUTHOR]

Thermal concentrators and cloaks with ellipsoidal shapes are designed by utilizing the transformation thermotics method and finite element simulations. The thermal conductivities for the concentrator and cloak are directly derive in Cartesian coordinates. The simulation results show that the ellipsoidal thermal concentrator can focus heat flux into a central region and that the ellipsoidal thermal cloak can guide heat flux around the cloaked region smoothly without disturbing the external temperature distribution and heat flux. The present method can be extended to design arbitrarily shaped thermal metadevices with novel properties. [ABSTRACT FROM AUTHOR]

Inverse problems for a three-dimensional magnetostatic model arising in the design of axisymmetric multilayered shielding and cloaking devices are stated. Assuming that the designed device consists of a finite number of spherical layers, each filled with a homogeneous isotropic medium, an optimization-based numerical algorithm is proposed for solving the problems. As a result, the considered inverse problems are reduced to finite-dimensional optimization ones in which the role of controls is played by the magnetic permeabilities of each elementary layer. The desired controls are found by applying particle swarm optimization. By analyzing numerical results, it is shown that the obtained optimal solutions correspond to cloaking devices having the highest efficiency in the considered class of devices and provide the simplicity of technical implementation. [ABSTRACT FROM AUTHOR]

Hitherto acoustic cloaking devices, which conceal objects externally, depend on objects' characteristics. Despite previous works, we design cloaking devices placed adjacent to an arbitrary object and make it invisible without the need to make it enclosed. Applying sequential linear coordinate transformations leads to a non-closed acoustic cloak with homogeneous materials, creating an open invisible region. Firstly, we propose to design a non-closed carpet cloak to conceal objects on a reflecting plane. Numerical simulations verify the cloaking effect, which is completely independent of the geometry and material properties of the hidden object. Moreover, we extend this idea to achieve a directional acoustic cloak with homogeneous materials that can render arbitrary objects in free space invisible to incident radiation. To demonstrate the feasibility of the realization, a non-resonant meta-atom is utilized which dramatically facilitated the physical realization of our design. Due to the simple acoustic constitutive parameters of the presented structures, this work paves the way toward realization of non-closed acoustic devices, which could find applications in airborne sound manipulation and underwater demands. [ABSTRACT FROM AUTHOR]

This article applies a new method to analyze modes in empty waveguides having perfectly conducting walls and a rather generic cross‐section. The method is based on a coordinate transformation that maps the generic profile of the waveguide cross‐section to a circular waveguide (CW) filled with an anisotropic material. Modes in this latter waveguide can be represented as a series over the corresponding free‐space CW modes. This representation is particularly useful if used as a modal expansion over the port of a 2.5D or 3D device treated with a transformation optics approach. The method is particularly efficient when the waveguide is "similar" to a CW. Applications are shown to the analysis of some waveguides of arbitrary cross‐section with a view on practical applications. [ABSTRACT FROM AUTHOR]

By extending the conventional scattering canceling theory, we propose a new design method for thermal cloaks based on isotropic materials. When the objects are covered by the designed cloaks, they will not disturb the temperature profile in the background zone. In addition, if different inhomogeneity coefficients are selected in the thermal cloak design process, these cloaks can manipulate the temperature gradient of the objects, i.e., make the temperature gradients higher, lower, or equal to the thermal gradient in the background zone. Therefore, thermal transparency, heat concentration or heat shield effects can be realized under a unified framework. [ABSTRACT FROM AUTHOR]

Metamaterials are manmade structures that have attained considerable attention over the past 2 decades in the modern fields like cloaking, sensing, and imaging owing to their ability to harness electromagnetic fields. In this regard, we have inspected the dielectric properties of the hyperbolic metamaterials (HMM) made of metallic nanorods and dielectric medium at the infrared wavelength regime. The periodically arranged subwavelength-sized metallic nanorods embedded in the silicon dioxide ( SiO 2 ) substrate glass. The spacing between two adjacent nanorods is of subwavelength in size. Furthermore, effective permittivity of the metamaterial has been analyzed by employing the Maxwell Garnett effective medium theory. Moreover, different metallic nanorod inclusions (Au, W, Pt and Ti) were taken into account. Hyperbolic features of the metamaterial have been observed, and it is noticed that hyperbolic features merely depend on the type of metallic nanorods and their size. The observations reveal that the dielectric properties of the metamaterial can be tuned by altering the radii of the metallic nanorod. Apart from dispersion, reflection, transmission and absorption features of the HMM are taken into account. It is noticed that reflection, transmission and absorption of the HMM depend on the type of metallic nanorod and its dimension. Such HMMs structures with exotic dielectric features would be useful for sensing, filtering, and absorption applications. [ABSTRACT FROM AUTHOR]

The commonly used coordinate transformation for cylindrical cloaks is generalized. This transformation is utilized to determine an anisotropic inhomogeneous diagonal material tensors of a shell type cloak for various material types, i.e., double-positive (DPS: [variant_greek_epsilon], μ > 0), double-negative (DNG: [variant_greek_epsilon], μ < 0), [variant_greek_epsilon]-negative (ENG), and μ-negative (MNG). To obtain conditions of perfect cloaking for various material types, a rigorous analysis is performed. It is shown that perfect cloaking will be achieved for same type material for the cloak and its surrounding medium. Moreover, material losses are included in the analysis to demonstrate that perfect cloaking for lossy materials can be achieved for identical loss tangent of the cloak and its surrounding material. Sensitivity of the cloaking performance to losses for different material types is also investigated. The obtained analytical results are verified using a Finite-Element computational analysis. [ABSTRACT FROM AUTHOR]

We derive a formula for the gradients of the total scattering cross-section (TSCS) with respect to positions of a set of cylindrical scatterers. The analytic form enhances modeling capability when combined with optimization algorithms and parallel computing. As application of the method, we consider a gradient-based minimization of TSCS for a set of cylindrical obstacles by incrementally repositioning them so that they eventually act as an effective cloaking device. The gradient-based optimization algorithm reduces the TSCS by evaluating its derivative with respect to the cylinder positions and then perturbatively optimizing the position of each cylinder in the cloaking device while taking into account acoustic multiple scattering between the cylinders. The method is illustrated for clusters of hard cylinders and sets of elastic thin shells in water. [ABSTRACT FROM AUTHOR]

In this paper, a piezoelectric cloaking mechanism is proposed, which makes the enclosed piezomagnetic cylinder invisible to elastic shear horizontal (SH) waves. Based on the scattering cancellation technique, the piezoelectric cloaking mechanism and dynamic stress concentration factor (DSCF) is obtained by the plane wave expansion method. A nonlinear ray trajectory equation for SH waves is derived based on the nonlinear transformation. Furthermore, piezoelectric effects on both cloaking mechanism and dynamic stress concentration are analyzed. The numerical results show that the scattering cancellation can be attributed to the cloak density, and the piezoelectric property can enhance the object's invisibility. The piezoelectric cloaking design can be applied to reduce the DSCF in some frequency regions, which means that it can change the stress distribution. It means that piezoelectric scattering cancellation can enhance both the cloaking results and structural strength of the mechanical metamaterials. This study is expected to have significance for the development and design of elastic wave metamaterials. [ABSTRACT FROM AUTHOR]

A model-based design allows representing complex, multi-domain systems as interconnected functional blocks, yielding graphical, intuitive information about the overall project, besides simplifying simulation. This work proposes using the modular approach as an optical engineering design and educational tool for developing paraxial ray optics setups, providing further integration with mechatronics subsystems and control loops. An expanded version of the ABCD transfer matrix modeling is implemented in MATLAB Simulink environment to simultaneously perform ray tracing and dynamic simulations. The methodology is validated for different problems, including paraxial cloaking, transmission through a multimode optical fiber, a Fabry–Perot interferometer, and an optical pickup with automatic focus, yielding reliable results with prospective applications in optical engineering design and for creating virtual labs devoted to multiphysics and mechatronics engineering courses. [ABSTRACT FROM AUTHOR]

Perpetua only saw martyrs in heaven, according to Tertullian, De anima 55,4. This passage has perplexed scholars, since Tertullian seems to be referring to Saturus's vision, not Perpetua's (Passio Perpetuae et Felicitatis 13,8). Additionally, Tertullian's citation is part of his larger argument against the Valentinians, in which he makes the peculiar claim that the souls of the dead are "below" (inferi) with the exception of the martyrs who are in Paradise. I contend that Tertullian's claim has been misunderstood in the last few decades of scholarship because of a failure to contextualize his remark within his rhetorical strategy. Disentangling Tertullian's convictions from his rhetoric is notoriously difficult, and yet by reading Tertullian as fully immersed in the tactics from the Second Sophistic Movement recent scholars have made great advances in our understanding of this North African Christian writer. Several of Tertullian's other works provide counter-evidence to the idea that only martyrs go to heaven: specifically, Tertullian further defines "heaven," its location, and its occupants; additionally, Tertullian clarifies who is a "martyr" in his wider oeuvre. When Tertullian's own teachings on the afterlife are retrieved, then one can re-read De anima to see how Tertullian has cloaked these with rhetorical devices meant to refute the Valentinian notion of the soul's ascent through multiple heavens. This idea that Tertullian believed only martyrs gain immediate access to heaven—which has often been repeated in the most recent century's secondary literature—is itself a misunderstanding of earlier modern scholarship. [ABSTRACT FROM AUTHOR]

This article aims to propose a novel analytical model for anisotropic multi-layer elliptical structures incorporating graphene layers. The multi-layer structure is formed of various magnetic materials, and each one has the permittivity and permeability tensors of $\overline {\overline {\varepsilon }}$ and $\overline {\overline {\mu }}$ , respectively. An external magnetic bias has been applied in the axial direction. A graphene layer, with isotropic surface conductivity of ${\sigma }$ , has been sandwiched between the two adjacent anisotropic materials. A novel matrix representation has been derived to find the propagation parameters of the multi-layer structure. Two exemplary important cases of the proposed general structure, as waveguides, have been investigated to show, first, the validity of our proposed analytical model and second, the richness of the general structure. The analytical and simulation results show an excellent agreement. A very large value of figure of merit (FOM), e.g., FOM = 110, is achieved for the second structure for the chemical potential and external magnetic bias of ${\mu }_{\text{c}}={0.9}$ eV and ${B}_{0}=1$ T, respectively. Our general structure and its analytical model can be exploited to design innovative terahertz devices such as absorbers, couplers, and cloaks. [ABSTRACT FROM AUTHOR]

Rendering an object invisible through a cloaking device is an ongoing dream of human beings and material scientists. Recently, intensive theories and experiments have predicted and demonstrated that such a cloaking device can be realized based on transformation optics and metamaterials, in fields of electromagnetics, optics acoustics, or even heat transfer. Metamaterials enable precise control over the propagation of waves due to their delicate micro-structures and spatially tailorable properties. However, a simple and natural way to achieve cloaking without a delicate micro-structure remains unattainable. Here, we report that an electromagnetic quasi-cloaking device can be readily achieved by a granular pile, formed by the falling-off and cumulation of particles on the object to be cloaked. We show that natural particle size segregation during the formation of the granular pile imparts the pile with a spatially variable filling ratio and resultant gradient-distributed permittivity, which enables the realization of the electromagnetic cloaking device. This work may open the possibility that a complicated electromagnetic device may be alternately achieved from manipulation of particle movement or arrangement. [ABSTRACT FROM AUTHOR]

This work investigates interfacial thermal transport in phononic‐mismatched heterostructures that consists of pristine black phosphorene and its phononic crystal. It is found that the presence of the sub‐periodic structure results in reduced thermal conductivity in phononic crystals. As opposed to intuitive expectations, a slight temperature jump is observed at the interface of nanophononic heterostructures, which is only about 10% of that at conventional interfaces consisting of dissimilar materials. Consequently, contact thermal conductance in the nanophononic heterostructure is 10−30 times higher than that of mass‐mismatched interfaces in a comparative study. Moreover, in contrast to conventional heterostructures achieved by interfacing dissimilar materials, weak temperature dependence is observed in interfacial thermal conductance, and thermal rectification is sharply suppressed. These phenomena are well explained based on lattice dynamic insights. This work not only enhances the understanding of the fundamental physics of phonons transport across interface, but also facilitates the possible spectrum of application ranges from thermoelectrics, thermal management, to thermal cloak. [ABSTRACT FROM AUTHOR]

Lubricant‐infused surfaces (SLIPSs/LISs) enable omniphobicity by reducing droplet pinning through creation of an atomically smooth liquid–liquid interface. Although SLIPSs/LISs provide efficient omniphobicity, the need for lubricant adds additional barriers to heat and mass transport and affects three‐phase contact line dynamics. Here, evaporation dynamics of microscale water droplets on SLIPSs/LISs are investigated using steady and transient methods. Although steady results demonstrate that evaporation on SLIPSs/LISs is identical to solid functional surfaces having equivalent apparent contact angle, transient measurements show significant increases in evaporation timescale. To understand the inconsistency, high‐speed optical imaging is used to study the evaporating droplet free interface. Focal plane shift imaging enables the study of cloaking dynamics by tracking satellite microdroplet motion on the cloaked oil layer to characterize critical timescales. By decoupling the effect of substrate material and working fluid via experiments on both microstructured copper oxide and nanostructured boehmite with water and ethanol, it is demonstrated that lubricant cloaking cannot be predicted purely by thermodynamic considerations. Rather, coalescence dynamics, droplet formation, and surface interactions play important roles on establishing cloaking. The outcomes of this work shed light onto the physics of lubricant cloaking, and provide a powerful experimental platform to characterize droplet interfacial phenomena. [ABSTRACT FROM AUTHOR]

Thermal metamaterials can effectively manipulate heat flux to achieve different thermal management functions, such as thermal cloak, concentrator, and rotator. To date, most of these metamaterials are based on macroscopic compound structures, such as metal/polymer. Herein, the concept of thermal metamaterials is extended to two‐dimensional (2D) graphene‐based systems because of their fast response speeds, in contrast to traditional three‐dimensional metamaterials. Three thermal metamaterials with heterogeneous thermal parameters are constructed using nano‐holed graphene, and some extraordinary thermal phenomena, such as effective shielding, accumulation and rotation of heat flux, are observed due to the significant anisotropic thermal conductivity of these 2D systems. Moreover, these thermal phenomena are insensitive to external disturbances. For designing thermal metamaterials, this study provides a novel approach, which can be applied to other 2D thermal functional materials. [ABSTRACT FROM AUTHOR]

Purpose: Modern fighter aircraft using active electronically scanned array (AESA) fire control radars are able to detect and track targets at long ranges, in the order of 50 nautical miles or more. Low observable or stealth technology has contested the radar capabilities, reducing detection/tracking ranges roughly to one-third (or even less, for fighter aircraft radar). Hence, infrared search and track (IRST) systems have been reconsidered as an alternative to the radar. This study aims to explore and compare the capabilities and limitations of these two technologies, AESA radars and IRST systems, as well as their synergy through sensor fusion. Design/methodology/approach: The AESA radar range is calculated with the help of the radar equation under certain assumptions, taking into account heat dissipation requirements, using the F-16 fighter as a case study. Concerning the IRST sensor, a new model is proposed for the estimation of the detection range, based on the emitted infrared radiation caused by aerodynamic heating. Findings: The maximum detection range provided by an AESA radar could be restricted because of the increased waste heat which is produced and the relevant constraints concerning the cooling capacity of the carrying aircraft. On the other hand, IRST systems exhibit certain advantages over radars against low observable threats. IRST could be combined with a datalink with the help of data fusion, offering weapons-quality track. Originality/value: An original approach is provided for the IRST detection range estimation. The AESA/IRST comparison offers valuable insight, while it allows for more efficient planning, at the military acquisition phase, as well as at the tactical level. [ABSTRACT FROM AUTHOR]

The invisible cloak has attracted great attention due to its significant influences on science and engineering. Active elastic wave metamaterials provide superior controllable characteristics for the flexural wave cloak that is obviously different from the conventional structure. In this work, a broadband flexural wave cloak is designed and fabricated by the coordinate transformation method and active control system. The cloaking configuration consists of 16 alternative concentric layers with the polydimethylsiloxane (PDMS) and piezoelectric (PZT) patches which are connected with the active control systems. The results of the finite element simulations and experiments show that the effective frequency range of the flexural wave cloak with the active control systems is extended from 900 Hz to 1200 Hz. The design and fabrication of the elastic cloak with broadband characteristics have shielding applications. [ABSTRACT FROM AUTHOR]

We study inverse problems of magnetostatics that arise when designing two-dimensional multilayered shielding and cloaking devices. It is assumed that the device to be designed has the form of a circular shell consisting of finitely many layers, each filled with a homogeneous isotropic medium. Using an optimization method, the inverse problems under consideration are reduced to finite-dimensional control problems in which the constant permeabilities of the layers play the role of control parameters. A numerical algorithm based on the particle swarm method is proposed for solving the control problems. Some properties of the algorithm proposed and of the resulting optimal solutions are established. Results of computational experiments are discussed. [ABSTRACT FROM AUTHOR]

We present the theoretical design, numerical simulation, and experimental demonstration of a single-parameter-based underwater ultrasound cloaking of arbitrary objects based on metagrating. The carpet metagrating is implemented with periodic grooves, which circumvents the tedious calculations and extreme material responses of the conventional cloaking based on acoustic transformation theory, providing a simple design methodology and enabling easy fabrication in real-life scenarios. Particularly, we expand the working frequency range of this ultrasound cloaking to 100–900 kHz, which is commonly used in biomedical ultrasound and industrial testing. Our design with the advantages of extreme simplicity, robust concealment of sizeable objects, and potential broadband functionality will improve the applicability of ultrasound carpet cloaking for more realistic situations where the camouflage of the arbitrary target is needed. [ABSTRACT FROM AUTHOR]

We propose a feasible design for an annulus-based dielectric metamaterial cloak to not only reduce Ohmic losses from metallic metamaterials but also avoid manual misalignment when utilizing discrete dielectric metamaterials. The designed cloak is authenticated by the finite-integration method, and the corresponding field distributions without and with hidden objects are found to reveal the invisibility of the cloak. In addition, illusion optics are comprehended by comparing the field distributions of the dielectric-metamaterial-cloak-based illusion-device and an illusion object. With the abovementioned characteristics of the proposed cloak using an annulus-based dielectric metamaterial, in simulation, we successfully approach the features of the reciprocal cloak, which conceals multiple objects with vision and movement. [ABSTRACT FROM AUTHOR]

This paper deals with a model for solids with porous channels filled by an incompressible isotropic fluid. The Darcy–Brinkman–Stokes law is derived, that represents a rate equation for the local mass flux of the fluid, presenting relaxation times in which this flux evolves towards its local-equilibrium value, viscous effects and a permeability tensor referring to a response of the system to an external agent, i.e. the fluid flow produced by a pressure gradient. The erosion/deposition phenomena in an elastic porous matrix are also studied and particular thermal porous metamaterials, that have interesting functionality, like in fluid flow cloaking, are illustrated as application of the obtained results. This derived model is completely in agreement with a theory formulated in the framework of the rational irreversible thermodynamics, where two internal variables are introduced (a symmetric structural porosity tensor and a symmetric second order tensor influencing viscous phenomena, that is interpreted as the symmetric part of the velocity gradient), when the results are considered in a first approximation and some suitable assumptions are done. The constitutive theory is worked out by using Liu's and Wang's theorems. The obtained theory has applications in several technological sectors, like physics of soil, pharmaceutics, physiology, etc., and contributes to the study of new porous metamaterials. [ABSTRACT FROM AUTHOR]

New connections between static elastic cloaking, low-frequency elastic wave scattering and neutral inclusions (NIs) are established in the context of two-dimensional elasticity. A cylindrical core surrounded by a cylindrical shell is embedded in a uniform elastic matrix. Given the core and matrix properties, we answer the questions of how to select the shell material such that (i) it acts as a static elastic cloak, and (ii) it eliminates low-frequency scattering of incident elastic waves. It is shown that static cloaking (i) requires an anisotropic shell, whereas scattering reduction (ii) can be satisfied more simply with isotropic materials. Implicit solutions for the shell material are obtained by considering the core–shell composite cylinder as a neutral elastic inclusion. Two types of NI are distinguished, weak and strong with the former equivalent to low-frequency transparency and the classical Christensen and Lo generalized self-consistent result for in-plane shear from 1979. Our introduction of the strong NI is an important extension of this result in that we show that standard anisotropic shells can act as perfect static cloaks, contrasting previous work that has employed 'unphysical' materials. The relationships between low-frequency transparency, static cloaking and NIs provide the material designer with options for achieving elastic cloaking in the quasi-static limit. [ABSTRACT FROM AUTHOR]

This paper is concerned with the polariton resonances and their application for cloaking due to anomalous localized resonance (CALR) for the elastic system within finite frequency regime beyond the quasi-static approximation. We first derive the complete spectral system of the Neumann-Poincaré operator associated with the elastic system in R 3 within the finite frequency regime. Based on the obtained spectral results, we construct a broad class of elastic configurations that can induce polariton resonances beyond the quasi-static limit. As an application, the invisibility cloaking effect is achieved through constructing a class of core-shell-matrix metamaterial structures provided the source is located inside a critical radius. Moreover, if the source is located outside the critical radius, it is proved that there is no resonance. [ABSTRACT FROM AUTHOR]

Purpose: In this paper, we present strategies for active vibration suppression and defect detection in a one-dimensional network of an arbitrary number of coupled spring–mass units connected in series. The choice of a spring–mass system is not arbitrary, as the latter is found in many applications throughout a wide range of fields, for instance in defense detection/shielding studies, biomedical engineering, structures engineering, computer graphics and acoustics among others. Methods: The system of differential equations that model the spring–mass systems was analyzed and solved using the Laplace transform and other analytic tools. The data used in the numerical simulations were obtained by solving the associated forward problems analytically or numerically. Some of the simulations required numerical integration and minimization routines. Results: A scheme for active vibration suppression is given via explicit formulas for the required control forces. The detect defection strategy is given in terms of an explicit formula whenever only the location or mass of a lone defect is unknown and in terms of a minimization procedure whenever more than one information about the defect(s) are unknown. Several numerical simulations were done to validate these results. Conclusion: As we show in the paper, the success of the vibration suppression scheme we developed depends on the speed and accuracy of the intervening active controls. Meanwhile, the defect detection algorithm only requires measurements in a sufficiently large time interval of the longitudinal vibrations in the first mass. [ABSTRACT FROM AUTHOR]

This work is devoted to studying a direct and inverse scattering problem by an elliptical void in a thermoelastic layer subjected to a dynamic load. In the context of the linear theory of thermoelasticity, coupled boundary integral equations, in terms of the displacement field and temperature over the contour of the void, are formulated. By measuring the surface oscillations, over a finite-length interval on the upper surface layer, the inverse problem in one mode-frequency range is constructed. So, we have developed an efficient numerical algorithm to identify the size and position of the void in the layer. A series of numerical examples, to check the influence of the thermoelastic properties on the algorithm of detection, has been presented at different noise levels. [ABSTRACT FROM AUTHOR]

Andromede is a new instrument for mass spectrometry analysis of nanodomains and nano-objects present on a surface. The molecular sample information (mass and structure) is obtained from time-of-flight spectra of secondary ions emitted under the impact of nanoparticles (NPs), as A u n q + with n/q up to 200 accelerated by a 4 MV single stage electrostatic Pelletron® accelerator. Such projectiles, providing huge emission rates per impact, permit the topographic analysis of complex samples. The energy deposition of these massive gold NPs induces surprisingly large holes of about 100 nm diameter in very thin graphene foils and average sputtered volume per impact of about 106 nm3 in a thick polymer material. The use of such massive clusters associated with multianode detectors, allowing the simultaneous detection of several ions of a given mass, permits correlation studies between the emitted ions within a single impact, shedding additional light on the chemical composition and structure of the analysed sample as illustrated here for various samples from metallic surfaces to biologic molecules deposits. [ABSTRACT FROM AUTHOR]

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Magnetic Janus particles (MJPs) have received considerable attention for their rich assembly behavior and their potential technological role in applications ranging from simple magnetophoretic displays to smart cloaking devices. However, further progress is hampered by the lack of predictive understanding of the cooperative self‐assembly behavior of MJPs and appropriate dynamic control mechanisms. In this paper, a detailed experimental and theoretical investigation into the magnetically directed spatiotemporal self‐assembly and switching of MJPs is presented. For this purpose, a novel type of MJPs with defined hemispherical compartments carrying superparamagnetic iron oxide nanoparticles as well as a novel simulation model to describe their cooperative switching behavior is established. Combination of the theoretical and experimental work culminates in a simple method to direct assemblies of MJPs, even at high particle concentrations. In addition, a magnetophoretic display with switchable MJPs is developed on the basis of the theoretical findings to demonstrate the potential usefulness of controlled large‐area assemblies of magnetic Janus particles. [ABSTRACT FROM AUTHOR]

Optical metamaterials consist of artificially engineered structures exhibiting unprecedented optical properties beyond natural materials. Optical metamaterials offer many novel functionalities, such as super-resolution imaging, negative refraction and invisibility cloaking. However, most optical metamaterials are comprised of rigid materials that lack tunability and flexibility, which hinder their practical applications. This limitation can be overcome by integrating soft matters within the metamaterials or designing responsive metamaterial structures. In addition, soft metamaterials can be reconfigured via optical, electrical, thermal and mechanical stimuli, thus enabling new optical properties and functionalities. This paper reviews different types of soft and reconfigurable optical metamaterials and their fabrication methods, highlighting their exotic properties. Future directions to employ soft optical metamaterials in next-generation metamaterial devices are identified. [ABSTRACT FROM AUTHOR]

Here, a two‐dimensional (2D) hole‐type square lattice photonic crystal is shown to achieve invisibility cloaking based on all‐angle self‐collimation. The proposed structure, which is composed of the high‐refraction‐index dielectric material PbTe (nPbTe ≈ 6), is applicable in the mid‐infrared (mid‐IR) frequency range. The cloaking region is capable of hiding any object of any shape and size since the incoming wave does not interact with the cloaked object. The optimization process and the functionality of the proposed structure are investigated by equal frequency contour analysis and the finite difference time domain (FDTD) method. [ABSTRACT FROM AUTHOR]

The propagation of acoustic waves is a fundamental topic in shallow ocean acoustics. We numerically demonstrate a three-dimensional zone of silence consisting of a circular tube with gradient index metamaterials attached to its rigid wall. The cloaking effect is verified by fine agreement with analytical calculations. [ABSTRACT FROM AUTHOR]

An analytical solution exists for a circular bilayer cloak against a background of linearly varying temperature distribution and under steady state. In particular, the ratio of the outer-layer and background conductivity is fixed, given the inner and outer radius of the outer layer. This conductivity ratio could be large, thus limiting the selection of bulk materials for the outer layer. This study reports a different design by adding a third layer of known conductivity between the inner and the outer layer of the bilayer cloak. The analysis extends the analytical solution of the bilayer cloak. For the same outer layer geometry between the bilayer and trilayer cloak, the proposed trilayer design gives a less limiting conductivity ratio between the outer layer and the background material than the bilayer design. Consequently, the trilayer design can enhance the selection of natural materials for the outer layer of the thermal cloak. [ABSTRACT FROM AUTHOR]

In the giga-Hz (GHz) regime, not only does an advanced electromagnetic platform need a stealth function to shield itself from radar detection, similar to the "invisible" cloak in the visible light regime, but also an enhanced transmission function to communicate with others. To satisfy the seemingly contradictory requirements, we here propose a multi-functional mode of plasma cloaking, invisible to radar detection but signal-enhanced in communication bands. Such a synergistic effect is achieved by combining a "detour" layer of cloaking and an enhancement layer of communication. Thus, with such a design we can successfully apply an ideal "cloak" to the low frequency detecting regime (in the P-band) and improve signal transmission capabilities in a higher frequency communication regime (in the L-band) at the same time. Furthermore, the tunability and limitations of the multi-functional system are discussed, and the principles of designing inner and outer plasma layers are acquired. [ABSTRACT FROM AUTHOR]

Here, we investigate a wearable-based IR and thermal stealth structure that effectively blocks IR and thermal radiation from a human body or device using a polyurethane–antimony tin oxide (PU–ATO) composite fiber. The aging time of the ATO sol prepared by a sol–gel method, and the concentration of ATO with respect to that of the PU matrix were optimized to prepare PU–ATO composite fibers that simultaneously have an appropriate mechanical strength (strength of ~4 MPa and strain of ~340%) and IR- and thermal radiation-shielding properties with ~98% IR light, as determined by Fourier transform IR spectroscopic studies. The fabricated PU–ATO composite fiber showed stable IR- and thermal radiation-shielding properties even when exposed to ten cycles of repeated temperature changes of −20 and +80 °C and long-term temperature changes for 30 days. In addition, the surface of the PU–ATO composite fiber was rendered hydrophobic to prevent the distortion of the IR and thermal radiation due to the wetting of the PU–ATO composite fiber with absorbed water. The PU–ATO composite fiber-based textile proposed herein can be applied in wearable IR- and thermal radiation-shielding technologies to shield IR signals generated by objects of diverse and complex shapes. Wearable technology: An infrared invisibility cloak A material that makes us invisible to infrared cameras has been developed by researchers in South Korea. Even in total darkness, the heat that our body produces can give away our position. Clothing that masks this infrared radiation to make the wearer indistinguishable from the background is therefore useful in stealth applications. Sang Yoon Park from Seoul National University, Sanghyun Ju from Kyonggi University, Suwon, and their colleagues have developed a fiber made from a combination of a polymer known as polyurethane and a metal oxide, antimony tin oxide (ATO). The fibers block thermal radiation because the ATO scatters infrared light. The team used a wet-spinning technique to create a number of different composite fiber with varying ATO concentrations so that they could optimize their useful lifetime. The polyurethane–antimony tin oxide (PU–ATO) composite fibers makes infrared (IR) radiations emitted by an object to be as similar as possible to ambient background radiation such that an IR detection sensor fails to distinguish the target object. Hydrophobic surface of the PU–ATO composite fiber prevent the distortion of the IR and thermal radiation caused by the wetting of the PU–ATO composite fiber with water. The PU–ATO composite fiber-based textile can be applied in wearable IR- and thermal radiation-shielding technologies to shield IR signals generated by objects of diverse and complex shapes. [ABSTRACT FROM AUTHOR]

Thermal cloaking and camouflage have attracted increasing attention with the progress of infrared surveillance technologies. Previous studies have been mainly focused on emissivity manipulation or using sophisticated thermal metamaterials. However, emissivity control is only applicable for objects that are warmer than the environment and lower emissivity is usually accompanied with high reflectance of the surrounding thermal signals if they have nonuniform temperature. Metamaterial‐based thermal camouflage holds great promise but their applications on human subjects are yet to be realized. Direct temperature control represents a more desirable strategy to realize dynamically adjustable camouflage within a wide ambient temperature range, but a wearable, portable, and adjustable thermo‐regulation system that is suitable for human subjects has not been developed. This work demonstrates a wearable and adaptive infrared camouflage device responding to the background temperature change based on the thermoelectric cooling and heating effect. The flexible thermoelectric device can realize the infrared camouflage effect to effectively shield the metabolic heat from skin within a wide range of background temperature: 7 °C below and 15 °C above the ambient temperature, showing promise for a broad range of potential applications, such as security, counter‐surveillance, and adaptive heat shielding and thermal control. [ABSTRACT FROM AUTHOR]

We present the design of an innovative wire antenna able to automatically hide or reveal its presence depending on the waveform of the received/transmitted signal. This unconventional behavior is achieved through the use of a novel waveform-selective cloaking metasurface exploiting a meander-like unit cell loaded with a lumped-element circuit capable to engineer the scattering of the antenna depending on the waveform of the impinging signal. Due to the time-domain response of the lumped-element circuit, the antenna is able switching its scattering behavior when interacts with either a pulsed wave (PW) or a continuous wave (CW) signal. The proposed configuration paves the way to a new generation of cloaking devices for intelligent antenna systems, extending the concept of antenna as a device capable to sense the external environment and change its electromagnetic behavior accordingly. [ABSTRACT FROM AUTHOR]

While great progress has been made in the theory and realization of invisibility devices, the issue of narrow-bandwidth operation remains unsolved. In particular, physical bounds have been recently derived, based on the Bode–Fano theory of broadband impedance matching, that quantify the inherent difficulty in broadening the bandwidth of any linear, causal, and passive cloak. If we relax the passivity restriction, introducing active media into the system, it is then relevant to ask the following questions: 1) Can the Bode–Fano limit on broadband invisibility be surpassed, at least theoretically, and to what extent? 2) Can we guarantee the stability of a broadband active cloak? In this article, we discuss some steps toward answering these general questions. We first offer an intuitive discussion of why a passive cloak cannot be arbitrarily broadband. Then, we use a 1-D example to show, for the first time, that active cloaks have indeed the potential to operate beyond the Bode–Fano bandwidth limit. Finally, we design a simple 3-D active cloak that exhibits a much broader bandwidth than passive scattering-cancellation cloaks. All the designed active cloaks are stable, and we discuss some relevant conditions to ensure stability. Our results may pave the way for ultrabroadband invisibility devices. [ABSTRACT FROM AUTHOR]

The ability to tune the resonant frequency of a self‐assembled ultrasonic metamaterial with mesoscale spatial resolution, after fabrication, by up to 250% is demonstrated. This tunability is achieved by the microlensing‐enabled modification of nanocontact features, wherein the metamaterial resonant elements "dig in" to the substrate. In addition to tunability exceeding prior MHz–GHz frequency ultrasonic metamaterial examples, the system presented herein can be tuned after assembly at a spatial resolution commensurate with the laser spot's diameter. It is posited that these aforementioned advantages will enable a new class of ultrasonic gradient index devices, such as ultrasonic elastic wave cloaks, that can be manufactured in a scalable manner and then rapidly tuned. Finally, it is expected that this large tunability at ultrasonic frequencies will have broader application to areas including optomechanics, acoustoplasmonics, quantum‐mechanical oscillators, and adhesion control. [ABSTRACT FROM AUTHOR]

In this paper, using quasi-conformal mapping, a bi-functional coating layer is designed with the intention of both cloaking and directivity enhancement of an omnidirectional antenna. For TM external waves coming from a certain direction, the proposed coating layer conceals the inner objects. In addition to the cloaking performance, the designed coating layer plays the role of a metamaterial-based lens that dramatically enhances the directivity level of inner omnidirectional loopfamily antennas. To reach this goal, a proper coordinate transformation is elaborately utilized to transform the cylindrical wavefronts radiated from the antenna into semi-pure plane waves. With appropriate simplifications, the proposed coating layer turns into an isotropic meta-device, which is more suitable to be fabricated. To prove the feasibility of the implementation, an SRR-meander line meta-atom is designed to locally realize the required permittivity and permeability distribution of the bi-functional layer. Full-wave simulations are performed via COMSOL finite element solver to validate the cloaking effect and directivity enhancement of the proposed coating layer, at the same time. [ABSTRACT FROM AUTHOR]

In this paper, a new wearable (flexible) textile fabric-based 3D metamaterials absorber (MMA) structure is proposed. The proposed MMA was created from three layers; weft-knitted fabric, silicone, and plain weave fabric and then, it was simulated in Computer Simulation Technology (CST). It was obtained maximum absorption power (99.66%) at 9.38 GHz, and the average absorption power of 81-95% was obtained in the frequency range of 8 to 12 GHz depending on the incident angle in the simulation. The effect of the wearable textile structure on absorption was investigated. When compared with other materials, it can be said that the proposed MMA is broadband, incident angle independent, TE and TM polarization-independent, flexible, breathable, wearable, ease of fabrication, practical, low weight, and cost advantage. With this designed the textile fabric-based MMA, it can be obtained both low reflection coefficients and low transmission coefficients at broadband X-band frequencies. This provides a good solution for the cloaking of radar systems. [ABSTRACT FROM AUTHOR]

Progress in material science has allowed the control of different physical fields in a manner that would be unachievable with ordinary materials. The development of engineered materials (the so‐called metamaterials) to control the conductive heat flux has revolutionized the manner of manipulating thermal energy and allowed the possibility of guiding the heat flux in ways difficult to accept for human intuition. Herein, a brief review regarding the recent progress and developments in thermal metamaterials conceived to perform several conductive heat flux manipulation tasks is provided. Deeply theoretical discussions concerning design methodologies are left a side, and the focus is made on the design and manufacturing feasibility of metamaterials for thermal cloaking and camouflage, heat flux concentration, and inversion, among other applications that lead to the next generation of thermal devices. [ABSTRACT FROM AUTHOR]

The material parameters of a metamaterial (MTM) are determined by the transformation function used in the optical transformation. Some previously reported MTMs, such as the invisibility cloak, the field rotator, and the field concentrator, were designed by a linear transformation. Their impedance was matched to the background so that no reflection was found; however, the material parameters were mismatched to the background due to the linear transformation function. In the present work, the parameters were matched by using high-order polynomial functions as the transformation function. Since similar materials are filled in boundary cells of the finite-difference time-domain (FDTD) algorithm, the stair-casing error was reduced and the tolerance against boundary abrasion was increased. The frequency response of the proposed method was analyzed. The proposed method is applicable to MTM structures that have complex boundary shapes. In this work, circular and elliptic boundary shapes were considered as examples. [ABSTRACT FROM AUTHOR]