Acoustic Acoustic metamaterials, sometimes referred to as sonic or phononic crystals, are architected materials designed to manipulate sound waves or phonons in gases, liquids, and solids. By tailoring effective parameters such as bulk modulus (β), density (ρ), and in some cases chirality, they can be engineered to transmit, trap, or attenuate waves at selected frequencies, functioning as acoustic resonators when local resonances dominate. Within the broader field of
mechanical metamaterials, acoustic metamaterials represent the dynamic branch where wave control is the primary goal. Acoustic metamaterials control, direct and manipulate
sound in the form of
sonic,
infrasonic or
ultrasonic waves in
gases,
liquids and
solids. As with electromagnetic waves, sonic waves can exhibit negative refraction.—that couple stress with particle velocity and linear momentum with strain, known as Willis couplings. They are named after J. R. Willis, who predicted them using a dynamic homogenization method. Much of the recent interest in Willis couplings has been driven by their local form (the Milton–Briane–Willis equations Electro-momentum coupling provides a mechanism for wave manipulation similar to Willis coupling, with the added benefit of electrical tunability.
Structural Structural metamaterials are a type of
mechanical metamaterial that provide properties such as crushability and lightweight characteristics. Using
projection micro-stereolithography, microlattices can be created using forms much like
trusses and
girders. Materials four orders of magnitude stiffer than conventional
aerogel, but with the same density have been created. Such materials can withstand a load of at least 160,000 times their own weight by over-constraining the materials. A ceramic nanotruss metamaterial can be flattened and revert to its original state. While metamaterials derive their extraordinary properties from engineered micro- or nano-scale architectures that manipulate wave behaviour, metastructures operate at the macro-scale, leveraging geometric design and modular assembly to achieve multifunctional mechanical performance across larger systems. Fully bio-based composite and modular metastructure cells based on trussed geometry encompassing bamboo rods and plant-based polymer joints demonstrate scalable mechanical performance, supporting up to 700 kg in compression with a mass of only 30 g.
Thermal Typically, materials found in nature, when homogeneous, are thermally isotropic, meaning heat diffuses at roughly the same rate in all directions. Thermal metamaterials, as a subclass of
mechanical metamaterials, achieve anisotropic and tailored thermal responses through architected internal structures. The term arose around 2008, when Fan, Gao, and Huang demonstrated shaped graded materials with apparent negative thermal conductivity, and introduced the concept of a thermal cloak through transformation thermotics. By carefully designing their geometry at nano, micro, meso, or macro scales, these materials exhibit effective thermal conductivities not accessible in natural materials. Their classification as
mechanical metamaterials stems from the fact that their unusual thermal behavior arises from engineered structure rather than chemical composition. Examples include composites with highly aligned fibers, particle arrays, or carbon nanotubes, where directional organization enables controlled heat flow.
Nonlinear Metamaterials may be fabricated that include some form of
nonlinear media, whose properties change with the power of the incident wave. Nonlinear media are essential for
nonlinear optics. Most optical materials have a relatively weak response, meaning that their properties change by only a small amount for large changes in the intensity of the
electromagnetic field. The local electromagnetic fields of the inclusions in nonlinear metamaterials can be much larger than the average value of the field. Besides, remarkable nonlinear effects have been predicted and observed if the metamaterial effective dielectric permittivity is very small (epsilon-near-zero media). In addition, exotic properties such as a negative refractive index, create opportunities to tailor the
phase matching conditions that must be satisfied in any nonlinear optical structure and can strongly modify the known nonlinear effects and enable new ones.
Liquid Metafluids offer programmable properties such as viscosity, compressibility, and optical. One approach employed 50-500 micron diameter air-filled
elastomer spheres suspended in
silicon oil. The spheres compress under pressure, and regain their shape when the pressure is relieved. Their properties differ across those two states. Unpressurized, they scatter light, making them opaque. Under pressure, they collapse into half-moon shapes, focusing light, and becoming transparent. The pressure response could allow them to act as a sensor or as a dynamic hydraulic fluid. Like
cornstarch, it can act as either a
Newtonian or a non-Newtonian fluid. Under pressure, it becomes non-Newtonian – meaning its viscosity changes in response to shear force.
Hall metamaterials In 2009,
Marc Briane and
Graeme Milton proved mathematically that one can in principle invert the sign of a 3 materials based composite in 3D made out of only positive or negative sign Hall coefficient materials. Later in 2015
Muamer Kadic et al. showed that a simple perforation of isotropic material can lead to its change of sign of the Hall coefficient. This theoretical claim was finally experimentally demonstrated by Christian Kern et al. In 2015, it was also demonstrated by Christian Kern et al. that an anisotropic perforation of a single material can lead to a yet more unusual effect namely the parallel Hall effect. This means that the induced electric field inside a conducting media is no longer orthogonal to the current and the magnetic field but is actually parallel to the latest. ==Frequency bands==