Thermal camouflaging metamaterials

Thermal camouflage technologies, which aim at blending the infrared (IR) signature of targets into the background to counter the IR detection, have witnessed increasing development. To achieve thermal camouflage, the rule of thumb is to balance the thermal radiation between the target and the background, and the corresponding conductive strategy is to tune the local temperature field while the radiative strategy is to tune the local emissivity. Following these two basic strategies, the thermal metamaterials and wavelength-selective emissivity engineering to achieve thermal camouflage are first introduced. Then the more advanced dynamic strategies are reviewed that can adapt to the varying environment under the external stimuli, like electricity, light, strain, chemical, wetting, temperature, etc. Particularly the phase-changing and bioinspired materials are presented and reviewed. Finally, critical considerations on the challenges and opportunities of next-generation thermal camouflage technologies are elaborated and four future directions are cast, including temperature-responsive emissivity engineering, soft materials, multispectral camouflage, and detection-feedback system. Overall, a detailed introduction to the working principle, the state-of-the-art progress, and the critical thinking on the future development on thermal camouflage technologies are presented.