Magnetic–acoustic biphysical invisible coats for underwater objects

Abstract

Magnetic fields and acoustic waves are the two fundamental measures to perceive underwater objects, which, however, have never been simultaneously handled before. In this work, we propose and demonstrate a biphysical submillimeter-thick metamaterial coat that can simultaneously make underwater objects invisible to both magnetic fields and acoustic waves. The conformal coat is a subtle integration of an open-cavity acoustic absorber made of a dissipative acoustic metasurface (AMS) and a bilayer magnetic cloak. Experimentally, a magnetic cloaking effect with a field disturbance ratio of <0.5% is obtained over a broad-frequency range (10-250 kHz), and the compound metamaterial coat can strongly attenuate ultrasonic waves with a near-unity absorptivity. The magnetic subcoat can be freely combined with various AMS layers to achieve a wideband acoustic stealth effect for different spectral regimes. This work may open up a new way to build multifunctional devices for various waterborne applications.

Ultrathin Conformal Magnetic Invisible Cloak for Irregular Objects

“…In the measurement, the sample is placed at the center of a pair of homemade Helmholtz coils (diameter = 20 mm) and 5 mm diameter Helmholtz coil connected with a phase-lock-in amplifier is used to inductively detect the local y -axis magnetic field component. The experimental details could be found in the previous publications. , From the simulation, we see that the relative field change ratio is comparatively very small and fluctuates with a tiny magnitude of less than 0.5% from ∼10 kHz to our maximum measured frequency 250 kHz limited by the phase-lock-in amplifier. The difference change Δθ is less than 0.5° in this range.…”

“…The experimental details could be found in the previous publications. 27,33 From the simulation, we see that the relative field change ratio is comparatively very small and fluctuates with a tiny magnitude of less than 0.5% from ∼10 kHz to our maximum measured frequency 250 kHz limited by the phase-lock-in amplifier. The difference change Δθ is less than 0.5°in this range.…”

“…In the quasi-static approximation, the bilayer cloak consisting of a ferromagnetic (FM)–diamagnetic composite has been mostly adopted to design a magnetic invisible device based on the algorithm of field “scattering” cancellation. The perfect diamagnetic property was originally realized by a superconductor − and later approximated by the dynamical screen effect of good metals enabling the room-temperature application. − The bilayer cloaking approach has been successfully extended to manipulate diffusive flux as well. − However, an analytical bilayer magnetic coat widely adopted before is easily applicable for objects only with either spherical or cylindrical geometries. − ,, Beyond these, the mathematical process quickly becomes tedious, particularly in dealing with metallic objects having sharp edges. From a practical point of view, this capability is fundamentally required.…”

“…27−32 However, an analytical bilayer magnetic coat widely adopted before is easily applicable for objects only with either spherical or cylindrical geometries. [18][19][20][21][22][23][24][25]27,33 Beyond these, the mathematical process quickly becomes tedious, particularly in dealing with metallic objects having sharp edges. From a practical point of view, this capability is fundamentally required.…”

Summary and Outlook

“…It is worth pondering how to make theoretical thermotics more enlightening and impact non-thermal fields. For example, the pioneering attempt to control multiphysical fields originates from theoretical thermotics (thermal plus DC fields [28]), which has been extended to wave control, such as electromagnetic, acoustic, plus water waves [29] and magnetic plus acoustic fields [30,31]. More research could be expected to extend the paradigms of theoretical thermotics to other non-thermal fields.…”