Ultrathin Conformal Magnetic Invisible Cloak for Irregular Objects

Abstract

Magnetic invisible cloaking has been previously demonstrated but only limited to objects with rotational geometries either in spherical or cylindrical shapes, for which the classic analytical bilayer scheme could be strictly applied to design the hiding coat. In this work, we show that a quasi-static cloaking effect could be achieved for irregular objects, e.g., metals with sharp edges, using a numerical optimization scheme. In the quasi-static limit, it is unambiguously proved that the disturbance of the irregular geometries could be well compensated by the inhomogeneous distribution of the soft ferromagnetic (FM) layer either in permeability values or in shapes under the framework of a bilayer cloak. An FM mesh coat with a constant thickness of 0.5 mm was successfully engineered to meet the specific requirements. Experimentally, good cloaking performance with a field disturbance of less than 0.5% has been achieved for a 2 × 2 × 5 cm3 brass bar in a wide frequency range from ∼10 to 250 kHz. A commercial metal scanner was also applied to verify the practical potential. The general strategy to hide almost arbitrary objects was discussed in the end. In principle, the numerical conformal coat engineered by the composite material proposed here could be broadly extended for objects with various geometries.

Hydrodynamic metamaterials: Principles, experiments, and applications

“…The value of k II / k III can be realized in porous medium fluid flow. As shown in Figure 6d, the thickness of the cloaking shell is R 2 − R 1 = 30 μm only, while the radius of the cloaked region is R 1 = 10 mm, resulting in the thickness ratio $${(R}_{2}-{R}_{1})/{R}_{1}$$ to be 0.003, which is about 10 times smaller than the previous ultrathin thermal cloak 72 and three orders of magnitude smaller than the existing hydrodynamic cloaks, 36 and thus sets a new record in the shell‐shaped cloak thickness 47,71 …”

“…69,70 However, the fabrication of such a complicated structure is still challenging. To overcome this difficulty, scattering cancelation theory has been developed and successfully used in electromagnetics, 71 acoustics, 31 direct current, 29 and heat conduction, 24,25 as well as hydrodynamics. 46,47 Scattering cancelation theory provides a simple way to realize cloaking and concentrating, in which homogenous and isotropic bulk material with normal parameters are adopted.…”

“…41 In 1 to be 0.003, which is about 10 times smaller than the previous ultrathin thermal cloak 72 and three orders of magnitude smaller than the existing hydrodynamic cloaks, 36 and thus sets a new record in the shell-shaped cloak thickness. 47,71 The length and time scales of hydrodynamic metamaterials…”

“…However, the fabrication of such a complicated structure is still challenging. To overcome this difficulty, scattering cancelation theory has been developed and successfully used in electromagnetics, 71 acoustics, 31 direct current, 29 and heat conduction, 24,25 as well as hydrodynamics 46,47 …”

Realizing the thinnest hydrodynamic cloak in porous medium flow

“…In an electromagnetic field, the bilayer cloak has achieved a dimensionless thickness of σ = 0.25. 22 In a direct current (DC) electric field, σ of an electric bilayer cloak is about 0.17. 23 In thermodynamics, researchers started to pursue the ultrathin cloak for the first time and reduced this ratio to an amazing value of 0.02.…”