Thermal Illusion Research Articles

Unprecedented Spatial Manipulation and Transformation of Dynamic Thermal Radiation Based on Vanadium Dioxide.

Reconfigurable infrared (IR) materials have widespread applications in thermal management and smart IR concealment. Although various reconfigurable IR materials can be customized by positive or negative differential VO2-based resonators, their insightful mechanism remains unknown. Here, we comprehensively investigate the fundamental design rule of reconfigurable thermal radiation between positive and negative differential thermal radiation properties for the first time. Importantly, the skin depth of VO2 film in the metal state is investigated to clarify the transformation from positive to negative differential thermal radiation properties, and the critical thickness is further derived, providing important guidance in designing the reconfigurable thermal radiation regulator. Furthermore, the reconfigurable multistate thermal images had been presented into one plate. The resulting emittance variation (△ε8-14μm) of the VO2-based resonator can change from 0.61 to -0.53, which consummates the ability for diverse demands such as infrared concealment, thermal illusion, and thermal management. This work constitutes a promising and universal route toward designing whole smart devices and may create new scientific and technological opportunities for platforms that can benefit from reconfigurable electromagnetic manipulation.

Thermal illusions for thermal displays: a review.

Thermal illusions, a subset of haptic illusions, have historically faced technical challenges and limited exploration. They have been underutilized in prior studies related to thermal displays. This review paper primarily aims to comprehensively categorize thermal illusions, offering insights for diverse applications in thermal display design. Recent advancements in the field have spurred a fresh perspective on thermal and pain perception, specifically through the lens of thermal illusions.

General three-dimensional thermal illusion metamaterials

Abstract Thermal illusion aims to create fake thermal signals or hide the thermal target from the background thermal field to mislead infrared observers, and illusion thermotics was proposed to regulate heat flux with artificially structured metamaterials for thermal illusion. Most theoretical and experimental works on illusion thermotics focus on two-dimensional materials, while heat transfer in real three-dimensional (3D) objects remains elusive, so the general 3D illusion thermotics is urgently demanded. In this study, we propose a general method to design 3D thermal illusion metamaterials with varying illusions at different sizes and positions. To validate the generality of the 3D method for thermal illusion metamaterials, we realize thermal functionalities of thermal shifting, splitting, trapping, amplifying and compressing. In addition, we propose a special way to simplify the design method under the condition that the size of illusion target is equal to the size of original heat source. The 3D thermal illusion metamaterial paves a general way for illusion thermotics and triggers the exploring of illusion metamaterials for more functionalities and applications.

Inverse Design and Experimental Verification of Metamaterials for Thermal Illusion Using Genetic Algorithms

Thermal metamaterials offer a promising avenue for creating artificial materials with unconventional physical properties, such as thermal cloak, concentrator, rotator, and illusion. However, designs and fabrication of thermal metamaterials are of challenge due to the limitations of existing methods on anisotropic material properties. We propose an evolutionary framework for designing thermal metamaterials using genetic algorithm optimization. Our approach encodes unit cells with different thermal conductivities and performs global optimization using the evolution-inspired operators. We further fabricate the thermal functional cells using 3D printing and verify their thermal illusion functionality experimentally. Our study introduces a new design paradigm for advanced thermal metamaterials that can manipulate heat flows robustly and realize functional thermal metadevices without anisotropic thermal conductivity. Our approach can be easily applied to fabrications in various fields such as thermal management and thermal sensing.

Upper Body Thermal Referral and Tactile Masking for Localized Feedback.

This paper investigates the effects of thermal referral and tactile masking illusions to achieve localized thermal feedback on the upper body. Two experiments are conducted. The first experiment uses a 2D array of sixteen vibrotactile actuators (4 × 4) with four thermal actuators to explore the thermal distribution on the user's back. A combination of thermal and tactile sensations is delivered to establish the distributions of thermal referral illusions with different numbers of vibrotactile cues. The result confirms that localized thermal feedback can be achieved through cross-modal thermo-tactile interaction on the user's back of the body. The second experiment is conducted to validate our approach by comparing it with thermal-only conditions with an equal and higher number of thermal actuators in VR. The results show that our thermal referral with a tactile masking approach with a lesser number of thermal actuators achieves higher response time and better location accuracy than thermal-only conditions. Our findings can contribute to thermal-based wearable design to achieve greater user performance and experiences.

Selective thermal emission and infrared camouflage based on layered media

Infrared camouflage based on artificial thermal metasurfaces has recently attracted significant attention. By eliminating thermal radiation differences between the object and the background, it is possible to hide a given object from infrared detection. Infrared camouflage is an important element that increases the survivability of aircraft and missiles, by reducing target susceptibility to infrared guided threats. Herein, a simple and practicable design is theoretically presented based on a multilayer film for infrared stealth, with distinctive advantages of scalability, flexible fabrication, and structural simplicity. The multilayer medium consists of silicon substrate, carbon layer and zinc sulfide film, the optical properties of which are determined by transfer matrix method. By locally changing the thickness of the coating film, the spatial tunability and continuity in thermal emission are demonstrated. A continuous change of emissive power is further obtained and consequently implemented to achieve thermal camouflage functionality. In addition, other functionalities, like thermal illusion and thermal coding, are demonstrated by thickness-engineered multilayer films.

Conditioned pain modulation is not associated with thermal pain illusion.

Paradoxical sensations, known as thermal pain illusions, can be evoked by painful cold-heat pulse stimulation. They may provide diagnostic value; however, the possible interaction between conditioned pain modulation and thermal pain illusions has not been explored. The present study examined: (1) whether conditioned pain modulation could be induced by alternating tonic painful cold-heat pulse stimulation; and (2) whether the presence of thermal pain illusions during the conditioning stimulus influences the degree of conditioned pain modulation. This study was approved by the Ethics Committee of Meikai University (A1507). Conditioned pain modulation was provoked using alternating painful cold-heat pulses delivered at 20-s intervals applied to the forearm. Thermal pain illusions were qualitatively evaluated, and conditioned pain modulation was assessed quantitatively using the pressure pain threshold as a test stimulus. Differences in the conditioned pain modulation effect between the participants who experienced thermal pain illusions and those who did not were analysed using Student's t-test. A significant positive conditioned pain modulation effect (51.0±4.7%, overall effect) was detected. There was no significant difference in conditioned pain modulation between the participants who experienced thermal pain illusions and those who did not (44.3±6.0% and 55.5±6.8%, respectively; p=0.255). Conditioned pain modulation induced by alternating painful conditioning cold-heat pulse stimulation was identical during the conditioning stimulation in volunteers with and without thermal pain illusions. Conditioning cold-heat pulse stimulation is useful to evaluate conditioned pain modulation. Moreover, conditioned pain modulation is not influenced by the presence of thermal pain illusions, indicating partially different underlying supraspinal, neuronal networks.

Thermal camouflages based on 3D thermal-null medium

Thermal camouflage devices, which can transfer the temperature field produced by the heat target on the object plane to pre-designed illusory temperature field on the image plane, are designed by a block building approach. The building blocks are four basic thermal components, which are designed based on the 3D thermal-null medium and can individually achieve the effects of thermal shifting, splitting, compression, and deforming. 3D thermal-null medium, which performs as a good ‘thermal fiber’ and can achieve directional heat conduction along its main axis, is designed and fabricated by placing specifically oriented copper-air arrays in polyurethane plates. The proposed method provides a simple way to design thermal camouflage devices, which can simultaneously camouflage the position, quantity, size and shape of the heat target by choosing/combining different basic thermal components. All designed thermal camouflage devices of different functions can be realized by different oriented copper-air arrays in polyurethane plates. Both numerical simulations and experimental measurements verify the performance of the designed thermal camouflage devices, which may have applications on thermal image processing and thermal illusion.

Tunable Thermal Pattern for Thermal Illusion and Encrypted Messaging

Recently, metamaterials which can actively tuning heat flux has attracted a lot of researchers and these thermal functional materials can be used in many thermal application scenarios such as encrypted messaging and thermal illusion. In this paper, the bilayer structure array with a chessboard-like structure has been proposed and we have numerically acquired a tunable thermal pattern based on effective medium theory. Through this strategy, a thermal pattern which is almost the same with the patterns of ‘ECE’ can be obtained. After quickly tuning this 3*12 array of the 4*4 chessboard-like bilayer structure, another thermal pattern which looks like the patterns of ‘SOS’ is observed in the infrared camera instead, demonstrating the feasibility of thermal illusion. Furthermore, the functionality of encrypted thermal messaging is also numerically demonstrated by resorting to storing the analog patterns of the binary numbers. Besides, this kind of tunable bilayer array is quite simple and flexible, and it can also serve as thermal metamaterial when the corresponding matching function is satisfied. In all, this present bilayer structure array with a chessboard-like structure can obtain tunable thermal patterns without disassembly of any components and it might open more strategies for creating novel thermal functionalities.

Reversible sequin fabrics as variable emittance surfaces

This study demonstrates, for the first time, the use of reversible sequin fabrics as inexpensive, scalable, and durable variable emittance surfaces. First, the spectral normal-hemispherical reflectance's of commercially available gold/black and silver/black reversible sequin fabrics were measured at room temperature for wavelength between 0.25 and 20 μm. The surface roughness and material composition of the sequins were also characterized. The dynamic range of the total hemispherical emittance of the gold/black sequin fabric at room temperature was found to be 0.8, exceeding the performance of other variable emittance surfaces reported to date. The use of reversible sequin fabrics for thermal management was demonstrated experimentally by achieving different equilibrium temperatures of the fabric exposed to simulated solar radiation by simply adjusting the sequin pattern. The experimental measurements were in good agreement with a first order transient thermal model. Finally, thermal infrared images were taken of the sequin fabrics to illustrate their capabilities for thermal camouflage, illusion, and messaging.

Flexible and high precision thermal metasurface

Thermal metamaterials are artificial materials used to manipulate heat flow in many applications, such as thermal protection, thermal camouflage, and precise temperature control. Most of the existing thermal metamaterials are mainly based on metal, which makes their fabrication complex and time-consuming, and limits their flexibility. Here, we show a strategy to simplify the fabrication process, improve machining accuracy, and realize flexibility in thermal metasurfaces. Our proposed thermal metasurface is fabricated by laser engraving of copper-graphene coating surface, utilizing graphene coating with high thermal conductivity instead of the traditional filling materials of low thermal conductivity. It maintains the integrity of copper substrate, giving the metasurface a good heat dispersion. Controlled temperature gradient patterns are established, and the metasurface can be bent without changing its features, except for a slight variation in its thermal gradient. Finally, its cloaking ability is demonstrated by camouflaging the same heat source in the shape of different objects. Our designed metasurface mitigates the limitations in design and fabrication of existing thermal metamaterials, and can be used in applications requiring large flexibility, thermal illusion, and large thermal gradients on small scales.

Thermal metadevices with geometrically anisotropic heterogeneous composites

The classical effective medium theory (EMT) provides a powerful tool in designing heterogeneous-composite functional thermal metadevices. But it is usually appropriate for spatially symmetric or geometrically isotropic systems, which limits the flexibility or adjustability in control. Here, we develop a generalized EMT to predict effective thermal conductivities of heterogeneous systems with elliptical (or ellipsoidal) inclusions. With the generalized EMT, omnidirectional thermal invisibility and directional Janus thermal illusion are designed and experimentally demonstrated just by tuning orientations of shape-anisotropy particles. In particular, we try to clarify the respective scopes of the generalized Maxwell-Garnett and Bruggeman theory based on both theoretical interpretations and numerical simulations. Our work may offer a promising fundamental framework and application prototype in elaborating thermal metadevices with asymmetry, irregularity, or anisotropy in configuration.

Illusion thermotics with topology optimization

Illusion thermotics, which aims at creating temperature fields to mislead a thermal detector, encompasses many thermal functionalities. It has attracted significant attention due to the increasing number of promising applications. Transformation thermotics and scattering cancellation methods are frequently adopted for illusion devices, though they have inherent defects such as anisotropic thermal properties and complicated fabrication methods. Here, we present a general framework for topology optimization, which can be used to design the structure of an illusion device automatically. The optimized configuration can camouflage the thermal exterior of an illusion device so that it is identical to a reference. The original heat source can drive the virtual heat source despite the structure being simply composed of natural materials. To establish an effective topology optimization model, a volume constraint was imposed on the total volume of iron and a prescribed objective function was used to evaluate the difference between the reference temperature and the temperature field during the topology optimization. The flexibility of the method presented was successfully validated by changing the total volume of iron, the number and locations of the thermal illusions, the types of composite materials, and the original heat sources. Topology optimization is a new and flexible approach for designing thermal metamaterials and metadevices for diverse thermal functionalities and beyond.

Omnithermal Restructurable Metasurfaces for Both Infrared-Light Illusion and Visible-Light Similarity

The existing challenges in designing infrared illusion are related to complex working environments, multifold heat-transfer modes, and fabrication difficulties. Recent research focuses mainly on solving one of these challenges. In this work, we consider them synthetically and propose a class of restructurable metasurfaces to show both illusion viewed in infrared light and similarity in when viewed in visible light. We consider the three basic modes of heat transfer (omnithermotics) in theoretical designs and use radiation-cavity effects in experimental manufacture. We also make it feasible to tune the surface temperature and emissivity synergistically. Besides, such metasurfaces can work in temperature-varying backgrounds and transient states. This scheme may not only provide a platform for the design of adaptable thermal illusion but may also show robustness under multifrequency detection.

Radiative metasurface for thermal camouflage, illusion and messaging.

Thanks to the conductive thermal metamaterials, novel functionalities like thermal cloak, camouflage and illusion have been achieved, but conductive metamaterials can only control the in-plane heat conduction. The radiative thermal metamaterials can control the out-of-plane thermal emission, which are more promising and applicable but have not been studied as comprehensively as the conductive counterparts. In this paper, we theoretically investigate the surface emissivity of metal/insulator/metal (MIM, i.e., Au/Ge/Au here) microstructures, by the rigorous coupled-wave algorithm, and utilize the excitation of the magnetic polaritons to realize thermal camouflage through designing the grating width distribution by minimizing the temperature standard deviation of the overall plate. Through this strategy, the hot spot in the original temperature field is removed and a uniform temperature field is observed in the infrared camera instead, demonstrating the thermal camouflage functionality. Furthermore, thermal illusion and thermal messaging functionalities are also demonstrated by resorting to using such an emissivity-structured radiative metasurface. The present MIM-based radiative metasurface may open avenues for developing novel thermal functionalities via thermal metasurface and metamaterials.

Thermal illusion with the concept of equivalent thermal dipole

The research on thermal illusion contributes to both fundamental theories and practical applications. In the existing literatures, the most common mechanism is to design a shell to disguise the inside core. However, the core-shell scheme may be weak to handle many-particle systems because N particles may require N specially-designed shells. This lacks efficiency and restricts practical applications. To solve this problem, we can no longer focus on the local effect of a single particle. In contrast, we should study the macroscopic effect of the N particles by treating each particle as an equivalent thermal dipole. Then, thermal illusion can be achieved when the macroscopic equivalent thermal dipole moments of different systems are equal to each other. This requires only once calculation and contributes to efficiency. Accidentally, the concept of equivalent thermal dipole helps to revisit the well-known Bruggeman theory and provides a clear physical image for it. The proposed scheme is verified by theoretical analyses, finite-element simulations, and laboratory experiments. Our work offers an efficient approach to achieving thermal illusion in many-particle systems, and contributes to potential applications in misleading infrared detection, manipulating heat flux, etc.

Thermal Perception and Thermal Devices Used on Body Parts Other Than Hand or Face.

Most fundamental research on thermal perception focuses on the fingers or the hand. Also most existing and proposed thermal devices are meant to be applied to hand or fingers. However, if the hands are needed for other tasks, application of thermal stimulation to other body regions should be considered. This paper surveys the literature on thermal perception and thermal devices relevant to such other body regions. It starts with a short description of the experimental methods used in the various studies, such as the methods of limits, the two-alternative forced choice method, and magnitude estimation. This is followed by thermal psychophysical studies on detection, adaptation, spatial summation, and resolution. Next some striking thermal illusions are presented, such as a thermal grill and a seemingly continuously warming or cooling stimulus. Finally, the few studies on thermal communication and applications are summarized. These latter studies mainly focus on communicating emotions or playing computer games. The overall conclusion of this survey is that thermal devices should not focus on conveying complex messages, but especially in the areas of gaming or communication there seem to be interesting possibilities for further developments.

Three-dimensional thermal illusion devices with arbitrary shape**Project supported by the National Natural Science Foundation of China (Grant No. 11702069) and the Fundamental Research Funds for the Central Universities, China (Grant No. HEUCFM180203).

Since the concept of invisible cloak was proposed by Pendry and Leonhardt in 2006, many researchers have applied the theory of coordinate transformation to thermodynamics and overcome the complexity of inhomogeneous and anisotropic of material parameters. However, only two-dimensional (2D) thermal illusion devices are researched recently. According to this situation, our study focuses on three-dimensional (3D) thermal illusion devices including shrinker (or invisible cloak), concentrator, amplifier, reshaper, and rotator with arbitrary shape in a general way. In this paper, the corresponding material parameters of thermal illusion devices mentioned above are derived based on the theory of transformation thermodynamics and the simulated results agree well with the theoretical derivations. In addition, the conventional invisible cloak just controls the temperature gradient rather than the temperature value which is more concerned in physical applications. Here, we find that the temperature value of the cloaked object can be controlled by adjusting the location of the original point of the coordinate system.

Three-Dimensional Illusion Thermotics with Separated Thermal Illusions

Three-Dimensional Illusion Thermotics with Separated Thermal Illusions

Thermal Illusion of Porous Media with Convection-Diffusion Process: Transparency, Concentrating, and Cloaking

Thermal Illusion of Porous Media with Convection-Diffusion Process: Transparency, Concentrating, and Cloaking