Keywords

1 Opening Remarks

Thermal diffusion, the process of heat transfer from regions of higher temperature to lower temperature, is crucial in various natural and engineered systems [1,2,3,4,5]. The ability to control and manipulate thermal diffusion has significant practical implications in various fields, including thermal regulation, sensing, and imaging technologies [6,7,8,9,10,11,12]. Metamaterials have emerged as a powerful tool for achieving unprecedented control over physical phenomena, including heat, sound, and electromagnetic waves [13,14,15,16,17,18,19]. These artificial structures are designed to exhibit unique properties not found in naturally occurring materials, allowing researchers to manipulate wave propagation, bend light, create perfect lenses, and even realize cloaking devices [20,21,22,23,24,25,26,27,28,29]. In the context of thermal diffusion, metamaterials offer opportunities to tailor and enhance heat transfer processes.

Omnithermal restructurable metasurfaces, a specific class of metamaterials, have attracted considerable interest and research attention [30, 31]. These metasurfaces have the potential to control thermal diffusion not only in the infrared domain but also in the visible-light regime [32,33,34,35]. This capability opens up a wide range of applications, including invisible thermal camouflage, anticounterfeiting measures, novel imaging techniques, and precise temperature regulation.

This chapter aims to explore the realm of omnithermal restructurable metasurfaces, focusing on their design principles, experimental demonstrations, and potential applications. It will investigate how these metasurfaces can be tailored to achieve thermal infrared illusions, where objects appear to have different temperatures than their actual states, and visual similarities, where objects mimic the appearance of other materials. Through a comprehensive analysis of theoretical models and experimental findings, the chapter aims to provide a deep understanding of the underlying physics and engineering strategies behind these remarkable materials.

Fig. 15.1
Four different panels each containing a different visual representation related to a thermal experiment. A has a grid pattern. B displays a thermal image of an object in pixelated form. C has the experimental setup with labeled components, and D presents a thermal map with temperature variations.

(from Ref. [2])

Acrylic plate drilled with a square lattice of holes representing thermal pixels and corresponding thermal conductivities.

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Recent advancements in infrared illusion have focused on two approaches: regulating surface temperatures and designing surface emissivities. For instance, Fig. 15.1 illustrates a human infrared thermogram captured using a digital thermal metasurface [2]. However, current research has limitations, as it often overlooks thermal convection and radiation in conductive systems, and surface structures remain distinguishable under visible light [36,37,38]. Tuning emissivities provides an alternative approach but requires phase-change materials and additional installations. The integration of both methods into a single platform presents challenges, as there is currently a lack of practical and synergistic solutions available.

Fig. 15.2
An infographic featuring the comparison between visible-light and infrared views of heat radiation and convection from a heat source, captured by an infrared camera.

(from Ref. [3])

Schematic illustration of the proposed thermal metasurface composed of three distinct arrays, generating unique images captured by an infrared camera and appearing similar in the visible light spectrum.

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To overcome these limitations, an omnithermal reconfigurable thermal metasurface is designed to simultaneously tune surface temperatures and emissivities on a single platform. By tailoring individual block units and assembling them into specific arrays, distinctive infrared patterns can be produced, as depicted in Fig. 15.2. The design incorporates all three modes of heat transfer: conduction, convection, and radiation, allowing for tailored surface temperatures and specific emissivities over a broad temperature range. This singular platform enables the symbiotic tailoring of both surface temperature and emissivity, achieving infrared illusions and visible-light similarities simultaneously. This chapter aims to uncover the potential of omnithermal restructurable metasurfaces in enhancing thermal diffusion, shedding light on their promising prospects for scientific advancement and technological innovation. Understanding the fundamental principles and capabilities of these metamaterials will pave the way for transformative progress in fields that rely on precise control of heat transfer processes.

The organization of this chapter is structured as follows: Firstly, it explores properties and design principles of omnithermal restructurable metasurfaces, emphasizing the consideration of all three heat transfer modes and the manipulation of surface temperature and emissivity. It then examines the experimental methods used to fabricate and characterize these metasurfaces, addressing the challenges in their practical realization. Finally, it discusses the potential applications of omnithermal restructurable metasurfaces, encompassing areas such as infrared illusion, visible-light similarity, anticounterfeiting measures, and temperature manipulation.

2 Theoretical Framework of Universally Thermo-Adjustable Metasurfaces

The theoretical framework of universally thermo-adjustable metasurfaces is based on the Stefan−Boltzmann law [39]. According to this law, the total thermal radiative energy density of a black body is proportional to the fourth power of the surface temperature. In the context of infrared imaging, the actual spectral radiance received by an infrared camera deviates from the ideal case described by the Stefan-Boltzmann law. This deviation is captured by the spectral directional emissivity. The spectral directional emissivity represents the ratio of an object’s actual spectral radiance to that of a black body with the same temperature and wavelength.

In most practical situations, a diffuse-emitter approximation is sufficient, and the surface emissivity can be simplified to depend only on the wavelength and surface temperature. However, if the surface temperature varies significantly across different parts of an object, the variation of emissivity with both wavelength and surface temperature needs to be taken into account.

To create controllable infrared illusions, the surface temperature and emissivity are independently tuned for each unit of the metasurface. The aim is to achieve the desired illusion by assembling these units in specific configurations. The tuning parameters include thermal conductivity, convective coefficient, radiative emissivity, and height of each unit, which correspond to the three modes of heat transfer. The temperature of the top surface \(T_\textrm{sur}\) for a steady state can be ascertained by applying the principles of heat flow conservation,

$$\begin{aligned} \boldsymbol{J}_\textrm{cond} = \boldsymbol{J}_\textrm{conv} + \boldsymbol{J}_\textrm{rad}, \end{aligned}$$
(15.1)

where \(\boldsymbol{J}_\textrm{cond}\), \(\boldsymbol{J}_\textrm{conv}\), and \(\boldsymbol{J}_\textrm{rad}\) represent the densities of conductive, convective, and radiative heat flow, respectively. The height of the unit is denoted as \(H_b\), while the thermal conductivity is represented by \(\kappa _b\). The surface is characterized by the convective coefficient \(h_b\) and the radiative emissivity \(\varepsilon _b\), respectively. Additionally, the temperatures of the heat source and the surrounding room are indicated by \(T_0\) and \(T_\textrm{air}\). \(\boldsymbol{J}_\textrm{cond}\), \(\boldsymbol{J}_\textrm{conv}\), and \(\boldsymbol{J}_\textrm{rad}\) can be formulated as indicated below:

$$\begin{aligned} J_\textrm{cond}&=\kappa _b\nabla T|_\textrm{bulk} =\kappa _b\frac{T_0-T_\textrm{sur}}{H_b},\end{aligned}$$
(15.2a)
$$\begin{aligned} J_\textrm{conv} &= h_b (T_\textrm{sur}- T_\textrm{air}),\end{aligned}$$
(15.2b)
$$\begin{aligned} J_\textrm{rad} &= \varepsilon _b \sigma (T_\textrm{sur}^4-T_\textrm{air}^4)\nonumber \\ &=\varepsilon _b \sigma (T_\textrm{sur}^2+T_\textrm{air}^2)(T_\textrm{sur}+T_\textrm{air})(T_\textrm{sur}-T_\textrm{air})\\ &=R_b(T_\textrm{sur}) (T_\textrm{sur}- T_\textrm{air}),\nonumber \end{aligned}$$
(15.2c)

where \(R_b(T)=\varepsilon _b \sigma (T_\textrm{sur}^2+T_\textrm{air}^2)(T_\textrm{sur}+T_\textrm{air})\) represents the radiative characteristic of the surface. By incorporating Eqs. (15.1–15.2c), we can derive the temperature of the top surface \(T_\textrm{sur}\)

$$\begin{aligned} T_\textrm{sur} = \frac{\kappa _b T_0/H_b+\left[ h_b+R_b(T_\textrm{sur})\right] T_\textrm{air}}{\kappa _b/H_b+h_b+R_b(T_\textrm{sur})}. \end{aligned}$$
(15.3)

The surface temperature can be predetermined to achieve a desired infrared illusion, and the remaining three parameters can be adjusted arbitrarily to attain the designed surface temperature. Tuning the surface emissivity becomes particularly important in cases where the surface temperature is nearly uniform within each unit, as it greatly influences the measured temperature.

Finally, these units are combined in a specific arrangement to generate the intended infrared illusion. Each unit serves as a pixel, and the contrast ratio of the imaging system relies on the highest and lowest reading temperatures. When the three modes of heat transfer are comparable, adjusting the surface temperature alone is enough to achieve the desired contrast ratio. However, if there is an imbalance in the heat transfer modes, adjusting the surface emissivity becomes necessary in order to create a discernible temperature distribution visible to the infrared camera.

The flexible arrangement of units enables reconfigurability without impacting the contrast ratio. Once the units are designed, the thermal metasurface consistently fulfills the resolution criteria of the infrared detector.

Fig. 15.3
A schematic of six groups of experiments with materials under different conditions of tuning conduction, convection, and radiation, accompanied by graphs plotting the variations in surrounding temperature with thermal conductivity, heat transfer coefficient, and emissivity.

(from Ref. [3])

Simulation results showing temperature distributions for different tuning methods of thermal conduction, thermal convection, and thermal radiation. Comparisons between theoretical and simulated surface temperature values are also illustrated.

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3 Finite-Element Simulation for Creating Infrared-Light Illusion and Visible-Light Similarity

To investigate the tuning of surface temperature, finite-element simulations were conducted using the commercial available software, COMSOL Multiphysics [40]. The computational simulations focused on adjusting thermal conductivity, convective coefficient, and radiative emissivity, while keeping the height fixed to maintain the geometric configuration of the metasurfaces. Initially, a metamaterial surface consisting of \(15\times 30\) units was created, where each unit was a 1 cm cube. These 450 units were divided into six groups, enabling the creation of six different patterns of , as shown in Fig. 15.3. The objective was to generate an illusion of the word “FUDAN”. The groups were then constructed together, as depicted in Fig. 15.3a. For simplicity, a uniform heat source was applied to the entire lower surface, and the surrounding air temperature was maintained at 300 K. The lateral surfaces of the metasurface were thermally connected to neighboring units to simulate realistic conditions. It can be observed that convection and radiation have minimal effects under low-temperature conditions, with thermal radiation having nearly indistinguishable influence.

The results demonstrate that by individually adjusting the three modes of heat transfer, the desired patterns can be achieved. Convection prevails at low temperatures, while radiation dominates at high temperatures, thereby exerting a substantial impact on the contrast ratio of the pattern observed through the infrared camera. Figure 15.3h–j provide a comparison between theoretical data and simulation results for temperature under the three tuning modes. These figures exhibit excellent agreement at low temperatures but exhibit slight deviations at high temperatures due to thermal interactions between different units.

4 Experimental Verification Using Cavity Effects

The influence of engineered emissivities on the apparent temperature distribution in infrared imaging can be observed through the surface-cavity effect [41, 42]. The presence of cavity structures on the surface allows for modulation of the surface emissivity, resulting in a deviation between the captured apparent temperature and the actual temperature, thereby creating illusive patterns. To design the surface cavity structure, a simplified cylindrical shape is adopted, as shown in Fig. 15.4a. The heat transfer process occurs between the surface cavity and the surrounding free space, allowing us to neglect the angle factor associated with the cavity. As stated in Ref. [41], the effective emissivity of an isolated cylindrical cavity relies on the area ratio between its opening and inner wall. Given the blocks’ regular shape and high thermal conductivities, we can assume a constant surface temperature. Since the plate surface only transfers energy into the environment, thermal interaction between cavities occurs exclusively. By manipulating the relative areas of the cavity and considering the inherent area ratio, one can tailor the effective emissivity of the surface. This customization allows for the creation of specific apparent temperature distributions in infrared imaging.

To experimentally investigate these effects, an FLIR E60 infrared camera with a resolution of 0.1 K is employed. For the experiments, a \(10\times 15\) array and two sets of customized units are utilized to design distinctive feature patterns. Copper cubes with a length of 2 cm are employed as block units, ensuring a homogeneous surface temperature due to copper’s high thermal conductivity of approximately 397 W m\(^{-1}\) K\(^{-1}\). Group I comprises solid blocks with an inherent emissivity of 0.2, whereas Group II contains blocks with cylindrical holes. The holes have a diameter of 0.8 cm and a depth of 1 cm. The effective emissivity for Group II is approximately 0.6. Additionally, an acrylic plate with a \(15\times 20\) array of square holes is designed to encode the block units and allow for their insertion and fixation. By manually rearranging these units, feature patterns depicting a human figure, a machine gun, and the letters “FD’ are created, as depicted in Fig. 15.4b–d. This process can also be mechanically automated using additional active installations, resulting in an actively reconfigurable metasurface. The encoded surface is placed in a water bath maintained at a temperature of 50\(\mathrm{^oC}\), while the room temperature remains around 20\(\mathrm{^oC}\). Once the system reaches a steady state, the infrared camera captures the feature patterns. The metasurfaces arranged in different configurations are challenging to distinguish in visible-light view (similarity). Furthermore, the robustness of the patterns is observed from various angles, both in infrared and visible-light views, as depicted in Fig. 15.4b–d. It should be noted that when an anti-reflection film is applied to the surface, the feature pattern disappears, and the recorded temperatures are slightly higher than before. This confirms the effectiveness of the cavity engineering method in modifying the imaging process.

Fig. 15.4
a. A diagram presents a cylindrical cavity in a cuboidal structure and its transition to a cuboidal structure. b to d are sets of photos and thermal images of a grid pattern surrounded by 2 temperature controllers. The thermal images at 0, 30, and 60 degrees present a human figure, machine gun, and letter F D-shaped patterns from b to d, respectively.

(from Ref. [3])

Experimental measurements demonstrating the effective emissivity principle using cavity structures and various patterns observed at different angles. The experimental setup is positioned within a temperature-controlled bath.

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5 Discussion and Application of Universally Thermo-Adjustable Metasurfaces

The imaging patterns captured by infrared cameras are influenced by the emissivity and surface temperature of objects. Exploiting this understanding, two tuning methods have been demonstrated on the same platform, either through simulation or experimentation, to accomplish infrared illusion and visible-light similarity. In the previous study [2], a feasible approach for manipulating conduction processes was proposed to tune the temperature. However, additional research is required to effectively regulate convective and radiative fluxes to align with the theoretical predictions mentioned earlier. The emissivity plays a dual role in the customization process: it directs the radiative flow, impacting the surface temperature, and it masks the actual temperature to deceive the infrared camera by presenting an apparent temperature. Therefore, the variable-controlling method is employed in the aforementioned simulations and experiments, demonstrating the flexibility and applicability of this platform for achieving infrared illusion.

Different tuning methods can be applied within various temperature ranges, and the encoding and assembling process of unit cells is non-destructive and replicable. The versatility in block assembly allows for the application of illusions in diverse situations. Furthermore, considering the limited dimensional resolution of infrared cameras, enhancing the quality of the illusion pattern is attainable when the dimensions of the units are on the same scale as the dimensional resolution. It should be noted that the proposed restructurability is fundamentally distinct from typical reconfigurability or adjustability [43]. This is because the former retains its properties but can be rearranged structurally, whereas the latter maintains its structure but allows for property adjustments. The suggested reconfigurable metasurface demonstrates both infrared-light illusion and visible-light similarity. In fact, the level of “similarity” can be enhanced to achieve “indistinguishability” with careful structuring of the surface, as shown in Fig. 15.4b–d. The feature holds significant potential for real-world applications.

One direct application of this scheme is for infrared anti-counterfeiting purposes. Anti-counterfeiting measures are widely employed across industries, military operations, and daily life. Conventional strategies rely on optical holograms [44,45,46], which can be detected either by the naked eye or specialized detectors. However, these technologies can be vulnerable as conventional patterns can be counterfeited. Recently, there has been a growing interest in the field of optical metasurfaces to address this challenge [47,48,49,50]. By designing two-dimensional microstructures, it becomes possible to tailor the amplitude, phase, and polarization of light arbitrarily, making the intrinsic signal characteristic and difficult to replicate. In such scenarios, the idea is to capture emissive electromagnetic-wave information exclusively for identification purposes. The intuitive approach is to tailor distinctive radiative signals to achieve anti-counterfeiting without requiring additional incident light. Encryption can be applied to the proposed metasurfaces, while decoding can be accomplished using infrared imaging. The key secret is challenging to forge due to its visual similarity in visible-light view. Furthermore, the reconfigurability of the metasurfaces enhances the level of difficulty for counterfeiting attempts. This anti-counterfeiting strategy finds extensive applicability in non-invasive and rapid-recognition scenarios.

6 Conclusion

In this chapter, we have presented a practical approach that enables the achievement of infrared-light illusion and visible-light similarity. By concurrently manipulating the surface temperature and emissivity, these parameters can be synergistically regulated. In comparison to existing thermal metamaterials, the proposed approach takes into account all three fundamental modes of heat transfer, thereby broadening the scope of potential applications. Furthermore, the introduction of the cavity effect facilitates the customization of emissivity, simplifying the manufacturing process. We believe that this approach not only tackles the challenges associated with designing infrared illusions but also holds promise for immediate applications in various industries and commercial sectors. With its unique capabilities and versatility, it has the potential to revolutionize thermal diffusion and find widespread usage in fields such as energy management, electronics, aerospace, and more. Further research and development in this direction can unlock even more exciting possibilities in the realm of metamaterial-based thermal control.