Tunable Metamaterials Research Articles

Integration of microfluidics in smart acoustic metamaterials

AbstractMicrofluidics has achieved a paradigm-shifting advancement in life sciences, automation, thermal management, and various other engineering streams. In recent years, a considerable amount of research has been conducted on the use of microfluidics in designing novel systems and fabricating next-generation smart materials that are capable of outperforming historical barriers and achieving unprecedented qualities. One such innovative development is the integration of fluidics into building artificially structured smart materials called acoustic metamaterials to achieve active tunability for a real-time controllable manipulation of acoustic waves. Leveraging the capability of microfluidics to automate the manipulation of liquid droplets, fluid streams, or bubbles in a required arrangement has revolutionised the development of actively tunable fluidics-integrated acoustic metamaterials for widescale applications. This review first discusses the prominent microfluidic actuation mechanisms used in the literature to develop fluidics-integrated smart acoustic metamaterials, and then it details integrated metamaterial design and extraordinary applications such as active acoustic wave manipulation or building tunable acoustic holograms etc. The following review concludes by providing the importance and future perspective of integrating microfluidic techniques with novel metamaterial designs, paving the way for innovative futuristic applications.

A bistable honeycomb mechanical metamaterial with transformable Poisson's ratio and tunable vibration isolation properties

With the increasing demand for vibration-isolation structures with tunable and multifunctional properties driven by the rapid development of complex engineering applications, metamaterials have emerged as promising candidates for this purpose owing to their extraordinary properties derived from their unique structural configurations. Owing to their unique deformation mechanisms, negative Poisson's ratio structures have been extensively studied in terms of vibration isolation and energy absorption; however, research on their performance tunability remains lacking. This paper describes a bistable honeycomb mechanical metamaterial (BHMM) consisting of cosine-shaped beams, which exhibits transformable Poisson's ratio and tunable vibration isolation properties. The proposed structure exhibited bistable properties, i.e., undergoing a configuration transition under external forces and retaining its existing configuration after the loading was removed. Furthermore, the two stable configurations exhibited distinctive elastic vibration characteristics, allowing the manipulation of elastic wave propagation on a subwavelength scale via local resonant mechanisms. Overall, this study presents a design methodology for multi-performance tunable metamaterials that satisfy the demands of the external environment. For example, the tunable vibration-isolation property of metamaterial makes it intriguing safety protection structures during crash events.

Electrostatically coupled tunable topological phononic metamaterials for angular velocity sensing

Electrostatically coupled tunable topological phononic metamaterials for angular velocity sensing

Continuously tunable mechanical metamaterials based on gear cells

Mechanical metamaterials with tunable mechanical properties have received widespread attention. However, it is still challenging to develop multifunctional mechanical metamaterials with continuous steady-state tunability over a wide range. In this work, a micro-gear unit structure is proposed, which is periodically arranged in a two-dimensional plane to form a metamaterial. The structural design principle and stability are analyzed. We show that this metamaterial can achieve continuous and stable adjustment of mechanical properties. Through finite element simulation, the study analyzes various mechanical properties of the metamaterial, including statics and dynamics. The results demonstrate that the Young's modulus of the external gear cells can be adjusted over 10 times, while the internal gear cells show an adjustment range of over 200 times. The shear modulus also exhibits significant adjustability. Additionally, this gear unit-based metamaterial allows for continuous changes in natural frequency and frequency amplitude curves, which presents new possibilities for vibration reduction material designs. Furthermore, the research presents multiple metamaterial structure designs based on this micro gear unit, showcasing its versatile application potential across various scenarios, offering broad prospects for practical applications.

The N-Oscillator Born–Kuhn Model: An In-Depth Analysis of Chiro-Optical Properties in Complex Chiral Systems

A comprehensive theory is developed for the chiral optical response of two configurations of the N-oscillator Born–Kuhn model (NOBK): the helically stacked and the corner stacked models. In the helical NOBK model, there is always a chiral response regardless of the value of N, whereas in the corner NOBK, only configurations with even N demonstrate a chiral response. Generally, the magnitudes of optical rotatory dispersion (ORD) and circular dichroism (CD) increase with N when the parameters of each oscillator are fixed. In cases of weak coupling, the spectral shapes of ORD and CD remain invariant, while strong coupling significantly alters the spectral shapes. For large damping, the spectral amplitude becomes smaller, and the spectral features become broader. In the presence of small damping, strong coupling introduces degeneracy in the coupled oscillator system, leading to multiple spectral features in both ORD and CD across the entire spectral region. This simple model can not only help in the design of tunable chiral metamaterials but also enhance our understanding of chiro-optical responses in structures with different configurations.

Tunable low-frequency wideband acoustic metamaterials with negative Poisson’s ratio and pre-compression

Tunable low-frequency wideband acoustic metamaterials with negative Poisson’s ratio and pre-compression

On-demand tunable metamaterials design for noise attenuation with machine learning

Metamaterials with structure-dominated properties provide a new way to design structures to obtain desired performance. To achieve a wide range of applications, on-demand tunable metamaterials would fulfill various and changing needs. The design of on-demand tunable metamaterials requires a higher-level understanding of the relationship between the properties of the metamaterials and the geometrical parameters, which in many cases are complicated and implicit. With the advancement of machine learning and evolutionary methods, it becomes possible to design on-demand tunable metamaterials. This paper designs on-demand tunable acoustic metamaterials for noise attenuation at varying frequencies by employing a genetic algorithm based neural network method. The C-shaped acoustic metamaterials with slidable shells are combined with the specifically designed tri-stable origami-inspired metamaterials to realize the on-demand tunable structure. Experiments were conducted and showed that the designed tunable metamaterials exhibited desired characteristics in different targeting frequency ranges. The present general methodology is expected to provide a route for on-demand tunable design while exploring more possibilities for the application of metamaterials.

Tunable Metamaterials for Impact Mitigation

AbstractTraditional methods of shielding fragile goods and human tissues from impact energy rely on isotropic foam materials. The mechanical properties of these foams are inferior to an emerging class of metamaterials called plate lattices, which have predominantly been fabricated in simple 2.5‐dimensional geometries using conventional methods that constrain the feasible design space. In this work, additive manufacturing is used to relax these constraints and realize plate lattice metamaterials with nontrivial, locally varying geometry. The limitations of traditional computer‐aided design tools are circumvented and allow the simulation of complex buckling and collapse behaviors without a manual meshing step. By validating these simulations against experimental data from tests on fabricated samples, sweeping exploration of the plate lattice design space is enabled. Numerical and experimental tests demonstrate plate lattices absorb up to six times more impact energy at equivalent densities relative to foams and shield objects from impacts ten times more energetic while transmitting equivalent peak stresses. In contrast to previous investigations of plate lattice metamaterials, designs with nonuniform geometric prebuckling in the out‐of‐plane direction is explored and showed that these designs exhibit 10% higher energy absorption efficiency on average and 25% higher in the highest‐performing design.

A novel anti-missing-rib lozenge lattice metamaterial with enhanced mechanical properties

In this work, a novel anti-missing-rib lozenge lattice metamaterial (ALLM) is designed. Besides, its mechanical properties are enhanced by introducing lateral and longitudinal auxiliary ribs. The mechanical properties and elasto-plastic deformation behaviors of the proposed metamaterials are systematically investigated through quasi-static compression experiments and finite element analysis. The results show that the designed ALLM exhibits distinct negative Poisson’s ratio (NPR) characteristics and superior specific energy absorption (SEA) capacity. The novel design concurrently enhances the stiffness while preserving the inherent NPR of ALLM. The effects of missing-rib lozenge subunit rotation angles, auxiliary rib configuration, and ligament width on the deformation patterns, energy absorption, stiffness, and NPR of the resulting metamaterials are investigated by parametric analysis. This study provides a reference for the design of tunable lozenge grid missing-rib lattice metamaterials.

In Situ Engineering of Conducting Polymer Nanocomposites at Liquid/Liquid Interfaces: A Perspective on Fundamentals to Technological Significance.

The conducting polymers have continuously been hybridized with their counterparts to overcome the intrinsic functional limitations compared to the metallic or inorganic analogs. Remarkably, the liquid/liquid interface-assisted methods represent an efficient and facile route for developing fully tunable metamaterials for various applications. The spontaneous adsorption of nanostructures at a quasi-two-dimensional interface is energetically favorable due to the reduction in interfacial tension, interfacial area, and interfacial energy (Helmholtz free energy). This Perspective highlights the fundamentals of nanostructure adsorption leading to hierarchical architecture generation at the interface from an experimentalist's point of view. Thereafter, the essential applications of the conducting polymer/nanocomposites synthesized at the interface emphasize the capability of the interface to tune functional materials. This Perspective also summarizes the future challenges and the use of the known fundamental aspects in overcoming the functional limitations of polymer/nanomaterial composites and also provides some future research directions.

Self-deployable contracting-cord metamaterials with tunable mechanical properties.

Recent advances in active materials and fabrication techniques have enabled the production of cyclically self-deployable metamaterials with an expanded functionality space. However, designing metamaterials that possess continuously tunable mechanical properties after self-deployment remains a challenge, notwithstanding its importance. Inspired by push puppets, we introduce an efficient design strategy to create reversibly self-deployable metamaterials with continuously tunable post-deployment stiffness and damping. Our metamaterial comprises contracting actuators threaded through beads with matching conical concavo-convex interfaces in networked chains. The slack network conforms to arbitrary shapes, but when actuated, it self-assembles into a preprogrammed configuration with beads gathered together. Further contraction of the actuators can dynamically tune the assembly's mechanical properties through the beads' particle jamming, while maintaining the overall structure with minimal change. We show that, after deployment, such metamaterials exhibit pronounced tunability in bending-dominated configurations: they can become more than 35 times stiffer and change their damping capability by over 50%. Through systematic analysis, we find that the beads' conical angle can introduce geometric nonlinearity, which has a major effect on the self-deployability and tunability of the metamaterial. Our work provides routes towards reversibly self-deployable, lightweight, and tunable metamaterials, with potential applications in soft robotics, reconfigurable architectures, and space engineering.

Actively tunable and switchable terahertz metamaterials with multi-band perfect absorption and polarization conversion.

In this paper, we theoretically present and numerically demonstrate an actively tunable and switchable multi-functional metamaterial based on vanadium dioxide (VO2) and graphene in the terahertz region. When VO2 is in the metallic phase, the proposed metamaterial serves as a multi-band perfect absorber, which exhibits the characteristics of insensitive polarization and robust tolerance for variations of the incidence angle. When VO2 is in the insulator phase, the proposed metamaterial acts as a polarization converter, which can simultaneously achieve perfect linear-to-linear and linear-to-circular polarization conversions. The simulation results show the cross-polarization conversion rate can reach ∼100% at the frequency region from 6.09 to 6.43 THz as well as 8.15 THz. Moreover, the ellipticity of linear-to-circular polarization conversion reaches ±1 at frequencies of 5.75 and 8.34 THz, respectively, which means the linear polarization waves can be completely converted into circular polarization waves. The proposed metamaterial provides new insight for the design of optoelectronic devices with multi-functionality in the terahertz region.

All-dielectric tunable zero-refractive index metamaterials based on phase change materials

Abstract We propose a tunable all-dielectric zero-index metamaterial based on phase change material. By constructing the alternating rod structure with phase change material and silica material, the zero-refractive index characteristic is realized. In order to verify the equivalent zero-index characteristic, we calculated the electromagnetic field distribution at center frequency. Based on phase invariance, it was found that this kind of metamaterial can achieve equivalent zero refractive index characteristics. To further achieve the polarization-insensitive zero refractive index, we propose a metamaterial structure based on the fishing nets structure. Based on the S-parameter inversion algorithm, the equivalent dielectric constant, equivalent permeability, equivalent impedance, and equivalent refractive index of the metamaterial were extracted. Based on the extracted equivalent parameters and the corresponding electromagnetic field distribution, the designed all-dielectric metamaterial has the tunable equivalent zero refractive index characteristic.

Longitudinal wave attenuation and bandgap analysis of a novel type hybrid tunable resonator metamaterial

Achieving vibration suppression or wave attenuation of tunable local resonance metamaterials without physical modification of the resonator has received a lot of attention recently. In this paper, a hybrid tunable metamaterial with a resonator containing a lever mechanism, an electromagnetic device, and a feedback system is proposed to achieve tunable bandgap and longitudinal wave attenuation. Unlike ordinary metamaterials, inertial amplification mechanisms and negative stiffness elements are involved in our proposed metamaterials. The function of the electromagnetic device and the feedback control are considered to theoretically derive the dispersion relation. The effective mass, the effective stiffness, the displacement transmission rate, and the band structure of the proposed metamaterial are analyzed, and the contributions of some key parameters on the bandgap are investigated. The accuracy of the analytical band structure is verified by the wave transmittance curves obtained by numerical simulation and the transient displacement response of a single oscillator. The results show that the proposed metamaterial yields broadband tunable bandgap in the low-frequency range, which enables the isolation of low-frequency longitudinal vibrations. This study provides a novel option for constructing integrated metamaterials to achieve vibration suppression and wave attenuation.

Controllable Wrinkling Inspired Multifunctional Metamaterial for Near‐Field and Holographic Displays

AbstractTunable electromagnetic (EM) metamaterials have received significant attention due to their compelling advantages of integration and minimization compared with conventional bulky devices. Meanwhile, mechanically reconfigurable metamaterials have witnessed a striving period over recent years due to their simplified structural composition and confined modulation capabilities. Here, a controllable‐wrinkling‐based reconfiguration method is proposed to design split‐ring resonant units with dynamic transmittance spectra by switching between planar and wrinkling morphologies. For the linear polarized incidence, the geometries of planar and wrinkled units are optimized to achieve on‐demanded manipulation of the phases and amplitudes, respectively. By simultaneously implementing the amplitude design and the phase gradient, the mechanically inspired metamaterial is engineered to display a near‐field and a holographic image. For circularly polarized incidences, the spin‐decoupled phases and chiral effects demonstrated in the planar and wrinkled state assist in designing a metamaterial to possess spin‐multiplexed and strain‐modulated fourfold displays. These results demonstrate the practical feasibility of the wrinkling method in engineering tunable metamaterials, and the design flexibility, as well as the mechanical strategy, can extend the potential toward the application scenarios such as information processing, sensing, imaging, flexible meta‐devices, etc.

A tunable slit-plate absorber with multiparameter cooperative control

Tunable acoustic metamaterials have a more flexible operating spectrum while still controlling acoustic waves via subwavelength thickness, but their operating range and acoustic absorption performance are limited by a single-parameter tuning mechanism. In this study, we first propose a slit-plate absorber with reconstructed porosity, which has a dynamic continuous tuning capability and is easier to achieve low-frequency resonance. Then, in order to improve its acoustic absorption performance, we study the tuning laws of numerous structural factors as well as their cooperative interaction. Local parameter changes to achieve impedance compensation and isotropic control to enhance frequency shift are presented to greatly improve the tunable range and sound absorption level. A tunable absorber with multi-parameter cooperative control is constructed using this method. The results reveal that continuous tuning of 210–920 Hz is possible at a total thickness of 25 mm, and the sound absorption coefficient in the tuning range is more than 0.8, and the thickness of the structure at 210 Hz is 1/65 of the absorption wavelength. Our work demonstrates a new method for improving the performance of adjustable acoustic metamaterials and promotes the use of acoustic metamaterials in low frequency changing noise environments.

Unlocking Optical Coupling Tunability in Epsilon‐Near‐Zero Metamaterials Through Liquid Crystal Nanocavities

AbstractEpsilon‐near‐zero (ENZ) metamaterials represent a powerful toolkit for selectively transmitting and localizing light through cavity resonances, enabling the study of mesoscopic phenomena and facilitating the design of photonic devices. In this experimental study, it demonstrates the feasibility of engineering and actively controlling cavity modes, as well as tuning their mutual coupling, in an ENZ multilayer structure. Specifically, by employing a high‐birefringence liquid crystal film as a tunable nanocavity, the polarization‐dependent coupling of resonant modes with narrow spectral width and spatial extent is achieved. Surface forces apparatus (SFA) allowed to continuously and precisely control the thickness of the liquid crystal (LC) film contained between the nanocavities and thus vary the detuning between the cavity modes. Hence, it is able to manipulate nanocavities anti‐crossing behaviors. The suggested methodology unlocks the full potential of tunable optical coupling in epsilon‐near‐zero metamaterials and provides a versatile approach to the creation of tunable photonic devices, including bio‐photonic sensors and/or tunable planar metamaterials for on‐chip spectrometers.

Local resonance bandgap control in a particle-aligned magnetorheological metamaterial

Stimuli-responsive elastic metamaterials augment unique subwavelength features and wave manipulation capabilities with a degree of tunability, which enables them to cut across different time scales and frequency regimes. Here, we present an experimental framework for robust local resonance bandgap control enabled by enhanced magneto-mechanical coupling properties of a magnetorheological elastomer, serving as the resonating stiffness of a metamaterial cell. During the curing process, ferromagnetic particles in the elastomeric matrix are aligned under the effect of an external magnetic field. As a result, particle chains with preferred orientation form along the field direction. The resulting anisotropic behavior significantly boosts the sensitivity of the metamaterial’s elastic modulus to the imposed field during operation, which is then exploited to control the dispersive dynamics and experimentally shift the location and width of the resonance-based bandgap along the frequency axis. Finally, numerical simulations are used to project the performance of the magnetically-tunable metamaterial at stronger magnetic fields and increased levels of material anisotropy, as a blueprint for broader implementations of in situ tunable active metamaterials.

Tunable 3D printed composite metamaterials with negative stiffness

The paper proposes a class of tunable metamaterials that use inclined beams to achieve instability in a rigid system. Three different beam tilt angles, 25°, 45°, and 60°, are evaluated in the form of unit cells using quasi-static compression tests and numerical simulations. Snap-through behavirous are characterised by structural stiffness and buckling load. Periodic and gradient structures are assembled and analysed by arranging the unit cells in rows and columns. Size effect analyses and parametric studies are carried out on various unit-cell arrangements and different beam angles. The proposed metamaterials are manufactured through fused filament fabrication 3D printing technology with a composite material, onyx. The results from experiments, finite element analysis, and analytical models are compared and evaluated. The structural stiffness and buckling load are shown to be positively related to the inclination angle of the tilted beams. The number of rows of unit cells governs the nonlinear mechanical response (number of snap-throughs) of multiple-layered structures. By increasing the number of rows and columns of unit cells, which are less prone to manufacturing defects, the reliability and repeatability of the structural properties of periodic/gradient structures could be improved. A design plot is also provided to predict and tune the snap-through behaviour of multiple-layered structures via beam angles and unit-cell arrangements.

Widely tunable magnetorheological metamaterials with nonlinear amplification mechanism

This paper proposes the design of an active nonlinear metamaterial for broadband and tunable low-frequency wave attenuation governed by a lever-based nonlinear amplification mechanism and a varying stiffness of a magnetorheological elastomer (MRE) driven by an external magnetic field. The metamaterial is formed by periodical spring-lever-MRE resonators connected to a base beam. Its strongly nonlinear behavior is described by a nonlinear amplification factor that depends on a magnetic flux density, a lever ratio, and an excitation amplitude. We use the Galerkin method to theoretically study the tunability of a low-frequency bandgap in terms of the MRE properties and the nonlinear amplification conditions. The obtained results are validated in numerical simulations. We also explore the mechanisms for broadband low-frequency wave attenuation governed by the strongly nonlinear dynamics of the proposed metabeam. Our results show that the metabeam with a large lever ratio can achieve substantial shifts of the bandgap frequencies for reasonably small variations of excitation amplitudes, while the bandgap size can be actively changed by varying the magnetic flux density. Our metamaterial is thus a promising solution to realize adaptive and actively tunable wave-attenuating metastructures that could be beneficial for practical applications.