Bioinspired Intelligent Ferrofluid: Old Magnetic Material with New Optical Properties.

Fine-tuning of microstructures enables the modulation of optical properties at multiple scales from metasurfaces to geometric optics. However, a dynamic system with a significant deformation range and topology transformation remains challenging. Owing to its magnetic controllability, ferrofluid has proven to be fertile ground for a wide range of engineering and technological applications. Here, we demonstrate a series of intelligent optical surfaces based on ferrofluid, through which multiple optical functions inspired by nature can be realized. The tunability is based on the topological transition of the ferrofluid between the flat state and cone array upon magnetic actuation. In the visible band, a tunable visual appearance is realized. In the mid-infrared band, active manipulation of reflection is realized based on the gradient-index (GRIN) effect. This system also features low latency response and straightforward manufacturability, and it may open opportunities for novel technologies such as smart windows, color displays, infrared camouflage, and other infrared-related technologies.

Ultralight Three-Layer Gradient-Structured MXene/ Reduced Graphene Oxide Composite Aerogels with Broadband Microwave Absorption and Dynamic Infrared Camouflage.

Infrared and radar detectors posed substantial challenges to weapon equipment and personnel due to their continuous surveillance and reconnaissance capabilities. Traditional single-band stealth devices are insufficient for dual-band detection in both infrared and microwave bands. To overcome this limitation, a gradient-structured MXene/reduced graphene oxide (rGO) composite aerogel (GMXrGA) is fabricated through a two-step bidirectional freeze casting process, followed by freeze-drying and thermal annealing. GMXrGA exhibits a distinct three-layered structure, with each layer playing a crucial role in microwave absorption. This deliberate design amplifies both the efficiency of microwave absorption and the material's effectiveness in dynamic infrared camouflage. GMXrGA displays an ultralow density of 5.2 mg∙cm-3 and demonstrates exceptional resistance to compression, enduring 200 cycles at a maximum strain of 80%. Moreover, it shows superior microwave absorption performance, with a minimum reflection loss (RLmin) of -60.1 dB at a broad effective absorption bandwidth (EAB) of 14.1 GHz (3.9-18.0 GHz). Additionally, the aerogel exhibits low thermal conductivity (≈26 mW∙m-1∙K-1) and displays dynamic infrared camouflage capabilities within the temperature range of 50-120 °C, achieving rapid concealment within 30 s. Consequently, they hold great potential for diverse applications, including intelligent buildings, wearable electronics, and weapon equipment.

Highly Thermally Conductive Aramid Nanofiber Composite Films with Synchronous Visible/Infrared Camouflages and Information Encryption.

The development of highly thermally conductive composites that combine visible light/infrared camouflage and information encryption has been endowed with great significance in facilitating the application of 5G communication technology in military fields. This work uses aramid nanofibers (ANF) as the matrix, hetero-structured silver nanowires@boron nitride nanosheets (AgNWs@BNNS) prepared by in situ growth as fillers, which are combined to fabricate sandwich structured thermally conductive and electrically insulating (BNNS/ANF)-(AgNWs@BNNS)-(BNNS/ANF) (denoted as BAB) composite films by "filtration self-assembly, air spraying, and hot-pressing" method. When the mass ratio of AgNWs@BNNS to BNNS is 1 : 1 and the total mass fraction is 50 wt %, BAB composite film has the maximum in-plane thermal conductivity coefficient (λ∥ of 10.36 W/(m ⋅ K)), excellent electrical insulation (breakdown strength and volume resistivity of 41.5 kV/mm and 1.21×1015â€…Ω â‹… cm, respectively) and mechanical properties (tensile strength of 170.9 MPa). 50 wt % BAB composite film could efficiently reduce the equilibrium temperature of the central processing unit (CPU) working at full power, resulting in 7.0 °C lower than that of the CPU solely integrated with ANF directly. In addition, BAB composite film boasts adaptive visible light/infrared dual camouflage properties on cement roads and jungle environments, as well as the function of fast encryption of QR code information within 24 seconds.

Three-Dimensional Electrochromic Soft Photonic Crystals Based on MXene-Integrated Blue Phase Liquid Crystals for Bioinspired Visible and Infrared Camouflage.

Developing bioinspired camouflage materials that can adaptively change color in the visible and infrared (IR) regions is an intriguing but challenging task. Herein, we report an emerging strategy for fabricating dynamic visible and IR camouflage materials by the controlled in situ growth of novel photopolymerizable blue phase liquid crystals with cubic nanoarchitectures onto highly aligned MXene nanostructured thin films. The resulting MXene-integrated 3D soft photonic crystals exhibit vivid structural colors and reversible switching between a bright colored state and a dark black state under a low DC electric field. As an illustration, proof-of-concept pixelated devices that allow for pixel-controllable electrochromism are demonstrated. Furthermore, a free-standing electrochromic flexible film of such 3D soft photonic crystals is fabricated, where visible electrochromism and thermal camouflage are enabled by leveraging the superior electrothermal conversion and low mid-IR emissivity of MXene nanomaterials.

Building-Envelope-Inspired, Thermomechanically Robust All-Fiber Ceramic Meta-Aerogel for Temperature-Controlled Dominant Infrared Camouflage.

The unsatisfactory properties of ceramic aerogels when subjected to thermal shock, such as strength degradation and structural collapse, render them unsuitable for use at large thermal gradients or prolonged exposure to extreme temperatures. Here, a building-envelope-inspired design for fabricating a thermomechanically robust all-fiber ceramic meta-aerogel with interlocked fibrous interfaces and an interwoven cellular structure in the orthogonal directions is presented, which is achieved through a two-stage physical and chemical process. Inspired by the reinforced concrete building envelope, a solid foundation composed of fibrous frames is constructed and enhanced through supramolecular in situ self-assembly to achieve high compressibility, retaining over 90% of maximum stress under a considerable compressive strain of 50% for 10 000 cycles, and showing temperature-invariance when compressed at 60% strain within the range of -100 to 500 °C. As a result of its distinct response to oscillation tolerance coupled with elastic recovery, the all-fiber ceramic meta-aerogel exhibits exceptional suitability for thermal shock resistance and infrared camouflage performance in cold (-196 °C) and hot (1300 °C) fields. This study provides an opportunity for developing ceramic aerogels for effective thermal management under extreme conditions.

Multiresonant Selective Emitter with Enhanced Thermal Management for Infrared Camouflage.

Tailoring the optical properties of metamaterials is crucial for improving the performance of infrared (IR) applications. Generally, IR camouflage materials are required to have low IR-emission properties for the detected bands (3-5 and 8-12 µm), in which IR detection is accomplished. However, the heat residue by suppressed thermal radiation degrades the thermal dissipation capacity and thermal stability of IR camouflage materials. Herein, a multilayer metal-dielectric-metal (MDM) selective emitter with high IR-emission performance in the undetected band for thermal management and low IR-emission performance in the detected band for IR camouflage is introduced. Compared to a conventional selective emitter and a low-emission material (Au film), the multiresonance selective emitter exhibited 125 and 2910% increases in heat dissipation within the undetected band, respectively. In addition, the proposed camouflage material exhibited a substantial reduction in emissive energy within the detected bands of 3-5 and 8-12 µm, with reductions of 72 and 83%, respectively, compared to that of a high-emission surface. The effectiveness of our IR camouflage was demonstrated by IR camera measurements. When the surface temperature was 360 K, the radiance temperatures of the multilayer multipeak selective emitter were 314 and 309 K for the 3-5 and 8-12 µm bands, respectively. Thermal management experiments demonstrated the enhanced thermal stability of the multiresonance selective emitter, especially in conditions of low pressure and high heat flux, when compared to that of the low-emissivity film. This work provides a practical strategy to enhance the thermal emission of a selective emitter, expanding its potential beyond IR camouflage to various energy applications.

Scalable, Color-Matched, Flexible Plasmonic Film for Visible-Infrared Compatible Camouflage.

The multispectral compatible infrared camouflage technology is implemented these days to counter the developing infrared detectors and detectors of other bands. However, the conflict between delicate optical structures and scalable procedures has significantly impeded the development and application of multispectral-compatible camouflage technology. Therefore, a semi-open Fabry-Perot structure is introduced, and the color and infrared emissivity by structural parameters for color-matched visible-infrared compatible camouflage are modulated. The prepared compatible camouflage film exhibits visible camouflage by the minimum color difference of 1.6 L*a*b* (under desert background) and infrared camouflage by low emission (ε3-5 µm ≈ 0.17 and ε8-14 µm ≈ 0.143). Due to its flexibility and scalability, the compatible camouflage film can be applied in practical applications and exhibits desirable visible and infrared camouflage performance in different battlefield backgrounds.

Three-Layered Thin Films for Simultaneous Infrared Camouflage and Radiative Cooling.

With the rapid advancements in aerospace technology and infrared detection technology, there are increasing needs for materials with simultaneous infrared camouflage and radiative cooling capabilities. In this study, a three-layered Ge/Ag/Si thin film structure on a titanium alloy TC4 substrate (a widely used skin material for spacecraft) is designed and optimized to achieve such spectral compatibility by combining the transfer matrix method and the genetic algorithm. The structure exhibits a low average emissivity of 0.11 in the atmospheric windows of 3-5 µm and 8-14 µm for infrared camouflage and a high average emissivity of 0.69 in 5-8 µm for radiative cooling. Furthermore, the designed metasurface shows a high degree of robustness regarding the polarization and incidence angle of the incoming electromagnetic wave. The underlying mechanisms allowing for the spectral compatibility of the metasurface can be elucidated as follows: the top Ge layer selectively transmits electromagnetic waves ranging from 5-8 µm while it reflects those in the ranges of 3-5 µm and 8-14 µm. The transmitted electromagnetic waves from the Ge layer are first absorbed by the Ag layer and then localized in the Fabry-Perot resonance cavity formed by Ag layer, Si layer and TC4 substrate. Ag and TC4 make further intrinsic absorptions during the multiple reflections of the localized electromagnetic waves.

Phase Change Microcapsule Composite Material with Intelligent Thermoregulation Function for Infrared Camouflage.

The infrared camouflage textile materials with soft and wear-resistant properties can effectively reduce the possibility of soldiers and military equipment being exposed to infrared detectors. In this paper, the infrared camouflage textile composites with intelligent temperature adjustment ability were prepared by different methods, using phase change microcapsule as the main raw material and high polymer polyurethane as the matrix, combining the two factors of temperature control and emissivity reduction. It was tested by differential scanning calorimeter, temperature change tester, infrared emissivity tester, and infrared imager. The results show that the temperature regulation effect of textile materials finished by coating method is better than dip rolling method, the temperature regulation ability and presentation effect are the best when the microcapsule content is 27%. When the bottom layer of infrared camouflage textile composite is 27% phase change microcapsule and the surface layer is 20% copper powder, its infrared emissivity in the band of 2-22 µm is 0.656, and the rate of heating and cooling is obviously slowed down. It has excellent heat storage and temperature regulation function, which can reduce the skin surface temperature by more than 6 °C and effectively reduce the infrared radiation. This study can provide reference for laboratory preparation and industrial production of infrared camouflage composite material. The infrared camouflage textile composite prepared are expected to be used in the field of military textiles.

One-Step Hydrothermal Synthesis, Thermochromic and Infrared Camouflage Properties of Vanadium Dioxide Nanorods.

Vanadium dioxide (VO2) has attracted interest from researchers because it undergoes a metal-insulator phase transition (MIT), which is accompanied by a reversible and remarkable change in both electrical and optical properties. VO2 exhibits numerous polymorphs and thus it is essential to control the growth of specific monoclinic VO2 (M) and rutile VO2 (R) phases. In this study, we developed a cost-effective and facile method for preparing VO2 nanorods with a highly crystalline monoclinic phase by one-step hydrothermal synthesis, in which only V2O5 and H2C2O4 are used as raw materials. The phase evolution of VO2 during the hydrothermal process was studied. The obtained VO2 nanorods were thoroughly mixed with fluorocarbon resin and homogeneous emulsifier in an ethanol solution to obtain a VO2 dispersion. To prepare VO2 films, screen printing was performed with a stainless steel screen mesh mask on glasses or fabric substrate. The VO2 coating had good thermochromic performance; the infrared transmittance change was greater than 20% @1.5 µm whilst keeping the visible transmittance greater than 50%. Meanwhile, the polyester base coating on the fabric had an emissivity change of up to 22%, which provides a solution for adaptive IR camouflage.

Freestanding Graphene Fabric Film for Flexible Infrared Camouflage.

Graphene films, fabricated by chemical vapor deposition (CVD) method, have exhibited superiorities in high crystallinity, thickness controllability, and large-scale uniformity. However, most synthesized graphene films are substrate-dependent, and usually fragile for practical application. Herein, a freestanding graphene film is prepared based on the CVD route. By using the etchable fabric substrate, a large-scale papyraceous freestanding graphene fabric film (FS-GFF) is obtained. The electrical conductivity of FS-GFF can be modulated from 50 to 2800 Ω sq-1 by tailoring the graphene layer thickness. Moreover, the FS-GFF can be further attached to various shaped objects by a simple rewetting manipulation with negligible changes of electric conductivity. Based on the advanced fabric structure, excellent electrical property, and high infrared emissivity, the FS-GFF is thus assembled into a flexible device with tunable infrared emissivity, which can achieve the adaptive camouflage ability in complicated backgrounds. This work provides an infusive insight into the fabrication of large-scale freestanding graphene fabric films, while promoting the exploration on the flexible infrared camouflage textiles.

Scalable-Manufactured Metamaterials for Simultaneous Visible Transmission, Infrared Reflection, and Microwave Absorption.

Scalable manufacturing of metamaterials with multispectral manipulation capabilities remains highly challenging, which was generally circumvented by integrating several single-spectral metamaterials, potentially leading to complex processes, large thicknesses, and limited fabrication size. We experimentally demonstrate a standalone and scalable-manufactured multispectral metamaterial featuring simultaneous visible transmission, infrared reflection, and microwave absorption. The prepared multispectral metamaterial with an area of 255 cm2 exhibits a visible transmittance of 74.5% at wavelengths of 400-700 nm (the highest 80.2% at 510 nm), a thermal emissivity of 0.08 at the infrared (IR) wavelengths of 2.5-20 µm (the lowest 0.03 at 19.5 µm), and a microwave absorptance of 63.4% at frequencies of 8.2-12.4 GHz (the near-perfect 97.4% at 11.5 GHz) on average with a deep-subwavelength thickness of λ/47. The deep-subwavelength multispectral metamaterial consists of a submillimeter-thick polyethylene terephthalate dielectric spacer sandwiched by a patterned ultrathin metal and a metal mesh back-reflector with ultralow sheet resistances. Unlike the conventional optically transparent microwave absorbers made from indium tin oxides, the surface plasmonic modes can be excited within the submillimeter-thick multispectral metamaterial, bringing about the gap plasmon polaritons-induced microwave attenuation, together with the excellent visible transparency and high IR reflection/low IR emissivity. This work may inspire the designs and practical production of standalone multispectral metamaterials and benefit the protection against ubiquitous IR and microwave reconnaissance without impeding visual observation.

Integrated Janus cellulosic composite with multiple thermal functions for personalized thermal management.

The effective integration of multiple thermal functions into one material is highly attractive in personal thermal management, taking the complex application environment into consideration. Herein, a multifunctional Janus cellulosic composite encompassing superior electrical heating, energy storage, thermal insulation, and infrared camouflage performance was firstly developed by integrating Janus cellulose nanofibers (CNF) aerogel, polypyrrole (PPy), and polyethylene glycol (PEG). In practice, the active heating-thermal regulation layer (PPy@CNFphilic-PEG) of multifunctional Janus cellulosic composite is faced inward to provide heating on-demand through the joint action of the electrically conductive PPy and thermally regulative PEG. The outward-facing hydrophobic aerogel layer (CNFphobic) serves as the thermal insulator, which simultaneously enables infrared camouflage by reducing heat loss to the environment via infrared radiation. This work presents an effective and facile strategy toward multifunctional Janus materials for efficient personal thermal management, showing great promise for potential applications, such as thermal comfort, infrared camouflage, and security protection.

Bioinspired Porous Anodic Alumina/Aluminum Flake Powder for Multiband Compatible Low Detectability.

Continuous development and advancement in modern detection technologies have increased the demand for multiband (e.g., visual and infrared) compatible camouflage. However, challenges exist in the requirements of incompatible structure resulting from the adaptation to different camouflage effects. This study is inspired by the light absorption structure of butterfly wing scales and demonstrates a porous anodic alumina/aluminum flake powder material prepared by a microscopic powder anodic oxidation technique for visual and infrared camouflage. The fabricated structures manipulate a compromise condition for visual camouflage by low reflectance (R̅400-800nm = 0.32) and dual-band infrared camouflage by low emission (ε̅3-5µm = 0.081 and ε̅8-14µm = 0.085). Further, the characteristic of short-range disorder in these bioinspired structures allows maintenance of the camouflage performance under omnidirectional detection (0-60°). This study provides new insight and a feasible method for coordinated manipulation of electromagnetic waves via bioinspired structural design and improved fabrication.

A Thermal Radiation Modulation Platform by Emissivity Engineering with Graded Metal-Insulator Transition.

Thermal radiation from a black body increases with the fourth power of absolute temperature (T4 ), an effect known as the Stefan-Boltzmann law. Typical materials radiate heat at a portion of this limit, where the portion, called integrated emissivity (εint ), is insensitive to temperature (|dεint /dT| ≈ 10-4 °C-1 ). The resultant radiance bound by the T4 law limits the ability to regulate radiative heat. Here, an unusual material platform is shown in which εint can be engineered to decrease in an arbitrary manner near room temperature (|dεint /dT| ≈ 8 × 10-3 °C-1 ), enabling unprecedented manipulation of infrared radiation. As an example, εint is programmed to vary with temperature as the inverse of T4 , precisely counteracting the T4 dependence; hence, thermal radiance from the surface becomes temperature-independent, allowing the fabrication of flexible and power-free infrared camouflage with unique advantage in performance stability. The structure is based on thin films of tungsten-doped vanadium dioxide where the tungsten fraction is judiciously graded across a thickness less than the skin depth of electromagnetic screening.

Metamaterial-Selective Emitter for Maximizing Infrared Camouflage Performance with Energy Dissipation.

Camouflage is a method evading predators in nature by assimilating into the environment. To realize an artificial camouflage surface for displays and sensors, many researchers have introduced several concepts including a metamaterial-selective absorber/emitter (MSAE). When an MSAE is adopted for camouflage at infrared (IR) wave, the energy dissipation of reduced emitting energy, as well as the reduction of emitting energy to deceive the IR signature from the surface, must be considered from the viewpoint of energy balance due to thermal instability. The integrated investigation of radiative heat-transfer characteristics and IR signature control of MSAE remains, however, poorly understood. Here, we investigate MSAE for IR camouflage by considering the energy balance in terms of reduction of emitting energy and dissipation of reduced emitting energy. On the basis of the atmospheric transmittance at an IR band, we designate the detected band as having wavelengths of 3-5 and 8-14 µm and the undetected band as having a wavelength of 5-8 µm. We investigate, via experiments and simulations, the optical characteristics required for IR camouflage and extract the factor that controls the emissive power. Furthermore, we suggest an integrated factor for evaluating the camouflage performance based on the concept of energy balance and propose a design guideline for MSAE with the aim of maximizing the camouflage performance at the IR band. This study will help to expand the range of applications (such as energy harvester and sensors) and others that are based on selective absorption/emission.