Mechanical metamaterials are artificial materials with mechanical properties that are defined by their mesostructure in addition to than their composition. They can be seen as a counterpart to the rather well-known family of optical metamaterials. They are often also termed elastodynamic metamaterials and include acoustic metamaterials as a special case of vanishing shear. Their mechanical properties can be designed to have values which cannot be found in nature. [1]
Acoustic or phononic metamaterials can exhibit acoustic properties not found in nature, such as negative effective bulk modulus, [2] negative effective mass density, [3] [4] or double negativity. [5] [6] They find use in (mostly still purely scientific) applications like acoustic subwavelength imaging, [7] superlensing, [8] negative refraction [9] or transformation acoustics. [10] [11]
Poisson's ratio defines how a material expands (or contracts) transversely when being compressed longitudinally. While most natural materials have a positive Poisson's ratio (coinciding with our intuitive idea that by compressing a material it must expand in the orthogonal direction), a family of extreme materials known as auxetic materials can exhibit Poisson's ratios below zero. Examples of these can be found in nature, or fabricated, [12] [13] and often consist of a low-volume microstructure that grants the extreme properties to the bulk material. Simple designs of composites possessing negative Poisson's ratio (inverted hexagonal periodicity cell) were published in 1985. [14] [15] In addition, certain origami folds such as the Miura fold and, in general, zigzag-based folds are also known to exhibit negative Poisson's ratio. [16] [17] [18] [19]
In a closed thermodynamic system in equilibrium, both the longitudinal and volumetric compressibility are necessarily non-negative because of stability constraints. For this reason, when tensioned, ordinary materials expand along the direction of the applied force. It has been shown, however, that metamaterials can be designed to exhibit negative compressibility transitions, during which the material undergoes contraction when tensioned (or expansion when pressured). [20] When subjected to isotropic stresses, these metamaterials also exhibit negative volumetric compressibility transitions. [21] In this class of metamaterials, the negative response is along the direction of the applied force, which distinguishes these materials from those that exhibit negative transversal response (such as in the study of negative Poisson's ratio).
A pentamode metamaterial is an artificial three-dimensional structure which, despite being a solid, ideally behaves like a fluid. Thus, it has a finite bulk but vanishing shear modulus, or in other words it is hard to compress yet easy to deform. Speaking in a more mathematical way, pentamode metamaterials have an elasticity tensor with only one non-zero eigenvalue and five (penta) vanishing eigenvalues.
Pentamode structures have been proposed theoretically by Graeme Milton and Andrej Cherkaev in 1995 [22] but have not been fabricated until early 2012. [23] According to theory, pentamode metamaterials can be used as the building blocks for materials with completely arbitrary elastic properties. [22] Anisotropic versions of pentamode structures are a candidate for transformation elastodynamics and elastodynamic cloaking.
Very often Cauchy elasticity is sufficient to describe the effective behavior of mechanical metamaterials. When the unit cells of typical metamaterials are not centrosymmetric it has been shown that an effective description using chiral micropolar elasticity (or Cosserat [24] ) was required. [25] Micropolar elasticity combines the coupling of translational and rotational degrees of freedom in the static case and shows an equivalent behavior to the optical activity.
In 2006 Milton, Briane and Willis [26] showed that the correct invariant form of linear elastodynamics is the local set of equations originally proposed by Willis in the late 1970s and early 1980s, to describe the elastodynamics of inhomogeneous materials. [27] This includes the apparently unusual (in elastic materials) coupling between stress, strain and velocity and also between momentum, strain and velocity. Invariance of Navier's equations can occur under the transformation theory, but would require materials with non-symmetric stress, [28] [29] hence the interest in Cosserat materials noted above. An elastostatic cloak with polar material with a distribution of body torque that breaks the stress symmetry was fabricated and successfully tested. [30] The theory was given further foundations in the paper by Norris and Shuvalov. [31] A mathematical theory of near cloaking for linear elasticity has been developed based on these papers. [32]
Meta-tribomaterials [33] [34] proposed in 2021 are a new class of multifunctional mechanical metamaterials with intrinsic sensing and energy harvesting functionalities. These material systems are composed of finely tailored and topologically different triboelectric microstructures. Meta-tribomaterials, a.k.a. self-aware composite mechanical metamaterials, can serve as nanogenerators and sensing media to directly collect information about its operating environment. They naturally inherit the enhanced mechanical properties offered by classical mechanical metamaterials. Under mechanical excitations, meta-tribomaterials generate electrical signals which can be used for active sensing and empowering sensors and embedded electronics. [33]
Electronic mechanical metamaterials [35] are active mechanical metamaterials with digital computing and information storage capabilities. They have built the foundation for a new scientific field of meta-mechanotronics (mechanical metamaterial electronics) proposed in 2023. [35] These material systems are an enhanced type of meta-tribomaterials created via integrating mechanical metamaterials, digital electronics and nano energy harvesting (e.g. triboelectric, piezoelectric, pyroelectric) technologies. They can sense the external stimuli, self-power and process the information to create an integrated closed-loop control system. Electronic mechanical metamaterials can be designed as digital logic gates, i.e., AND, OR, XOR, NAND, NOR, and XNOR, or mechanically-responsive data storage devices. Thus, they can potentially lead to developing future mechanical metamaterial computers (MMCs), complementing traditional electronics with electronics made of mechanical metamaterials. [35] Such computing metamaterial systems can be particularly useful under extreme loads and harsh environments (e.g. high pressure, high/low temperature and radiation exposure) where traditional semiconductor electronics cannot maintain their designed logical functions.
Another mechanism to achieve non-symmetric stress is to employ pre-stressed hyperelastic materials and the theory of "small on large", i.e. elastic wave propagation through pre-stressed nonlinear media. Two papers written in the Proceedings of the Royal Society A in 2012 established this principal of so-called hyperelastic cloaking and invariance [36] [37] and have been employed in numerous ways since then in association with elastic wave cloaking and phononic media.
Material systems have been developed that effectively achieve theoretical upper bounds for specific stiffness and strength. [38] [39] While theoretical composites that achieve the same upper bound have existed for some time, [40] they have been impractical to fabricate as they require features on multiple length scales. [41] Single length scale designs are amenable to additive manufacturing, where they can enable engineered systems that maximize lightweight stiffness, strength and energy absorption.
In theoretical physics, a roton is an elementary excitation, or quasiparticle, seen in superfluid helium-4 and Bose–Einstein condensates with long-range dipolar interactions or spin-orbit coupling. The dispersion relation of elementary excitations in this superfluid shows a linear increase from the origin, but exhibits first a maximum and then a minimum in energy as the momentum increases. Excitations with momenta in the linear region are called phonons; those with momenta close to the minimum are called rotons. Excitations with momenta near the maximum are called maxons.
In particle physics, a tetraquark is an exotic meson composed of four valence quarks. A tetraquark state has long been suspected to be allowed by quantum chromodynamics, the modern theory of strong interactions. A tetraquark state is an example of an exotic hadron which lies outside the conventional quark model classification. A number of different types of tetraquark have been observed.
A metamaterial is any material engineered to have a property that is rarely observed in naturally occurring materials. They are made from assemblies of multiple elements fashioned from composite materials such as metals and plastics. These materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Metamaterials derive their properties not from the properties of the base materials, but from their newly designed structures. Their precise shape, geometry, size, orientation and arrangement gives them their smart properties capable of manipulating electromagnetic waves: by blocking, absorbing, enhancing, or bending waves, to achieve benefits that go beyond what is possible with conventional materials.
Auxetics are structures or materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their particular internal structure and the way this deforms when the sample is uniaxially loaded. Auxetics can be single molecules, crystals, or a particular structure of macroscopic matter.
Negative refraction is the electromagnetic phenomenon where light rays become refracted at an interface that is opposite to their more commonly observed positive refractive properties. Negative refraction can be obtained by using a metamaterial which has been designed to achieve a negative value for (electric) permittivity (ε) and (magnetic) permeability (μ); in such cases the material can be assigned a negative refractive index. Such materials are sometimes called "double negative" materials.
Sir John Brian Pendry, is an English theoretical physicist known for his research into refractive indices and creation of the first practical "Invisibility Cloak". He is a professor of theoretical solid state physics at Imperial College London where he was head of the department of physics (1998–2001) and principal of the faculty of physical sciences (2001–2002). He is an honorary fellow of Downing College, Cambridge, and an IEEE fellow. He received the Kavli Prize in Nanoscience "for transformative contributions to the field of nano-optics that have broken long-held beliefs about the limitations of the resolution limits of optical microscopy and imaging.", together with Stefan Hell, and Thomas Ebbesen, in 2014.
A split-ring resonator (SRR) is an artificially produced structure common to metamaterials. Its purpose is to produce the desired magnetic susceptibility in various types of metamaterials up to 200 terahertz.
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Alexander Balankin is a Mexican scientist of Russian origin whose work in the field of fractal mechanics and its engineering applications won him the UNESCO Science Prize in 2005.
An acoustic metamaterial, sonic crystal, or phononic crystal is a material designed to control, direct, and manipulate sound waves or phonons in gases, liquids, and solids. Sound wave control is accomplished through manipulating parameters such as the bulk modulus β, density ρ, and chirality. They can be engineered to either transmit, or trap and amplify sound waves at certain frequencies. In the latter case, the material is an acoustic resonator.
A photonic metamaterial (PM), also known as an optical metamaterial, is a type of electromagnetic metamaterial, that interacts with light, covering terahertz (THz), infrared (IR) or visible wavelengths. The materials employ a periodic, cellular structure.
A seismic metamaterial, is a metamaterial that is designed to counteract the adverse effects of seismic waves on artificial structures, which exist on or near the surface of the Earth. Current designs of seismic metamaterials utilize configurations of boreholes, trees or proposed underground resonators to act as a large scale material. Experiments have observed both reflections and bandgap attenuation from artificially induced seismic waves. These are the first experiments to verify that seismic metamaterials can be measured for frequencies below 100 Hz, where damage from Rayleigh waves is the most harmful to artificial structures.
Theories of cloaking discusses various theories based on science and research, for producing an electromagnetic cloaking device. Theories presented employ transformation optics, event cloaking, dipolar scattering cancellation, tunneling light transmittance, sensors and active sources, and acoustic cloaking.
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Platonic crystals are periodic structures which are designed to guide flexural wave energy through thin elastic plates.
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Diffusion metamaterials are a subset of the metamaterial family, which primarily comprises thermal metamaterials, particle diffusion metamaterials, and plasma diffusion metamaterials. Currently, thermal metamaterials play a pivotal role within the realm of diffusion metamaterials. The applications of diffusion metamaterials span various fields, including heat management, chemical sensing, and plasma control, offering capabilities that surpass those of traditional materials and devices.