Thermal Illusion Research Articles

Digital thermal metasurface with arbitrary infrared thermogram

An object exhibits the infrared thermogram of another object, which is called thermal illusion as extensively investigated in the field of thermal metamaterials. However, almost all the existing thermal illusion behaviors were theoretically designed by using unconventional thermal conductivities, which means that the conductivities must be anisotropic, graded, or even singular due to the analytical methods in use. This problem largely limits fabrications for applications. By suggesting two discretization steps, here we put forward a numerical method instead to design thermal illusion, for which unconventional conductivities are no longer needed. In the meantime, more importantly, we reveal different thermal illusion behaviors. By tailoring the joint effects of thermal conduction and convection, we design a thermal pixel of cuboidal shape. We show that the assembly of such pixels into different two-dimensional arrays could generate infrared thermograms of different objects, which is thus called the digital thermal metasurface. Also, the metasurface is reconfigurable, and it can apparently produce all the existing thermal illusion behaviors reported in the literature. Finally, we experimentally fabricate a prototype. This work opens a door for applying conventional thermal conductivities of commercially available materials to thermal illusion, and we expect it to stimulate more exciting developments in electromagnetic disguise and confrontation.

Thermal illusion with twinborn-like heat signatures

Thermal illusion device, which can change an arbitrary object into another one when observed from an infrared vision, recently has attracted considerable attention amongst a series of innovative thermal phenomena by resorting to the unique thermal metamaterial. To enhance the illusion deceptiveness, current thermal illusion devices mostly change the location or shape of the target, but seldom deal with the number of heat signatures. In this paper, we develop a general formula to enhance the illusion deceptiveness by splitting the thermal target to appear like thermal bilocation with twinborn distinct heat signatures. The exhaustive design process based on transformation thermodynamics is rigorously introduced in order to realize thermal bilocation. Simulative and experimental results agree well with each other and validate the conception of thermal bilocation. Additionally, the influence of three structure parameters on the bilocation effects has been considered for further optimization. Our device can thermally camouflage the original target with twinborn signatures at different locations, improving the deceptiveness greatly compared with only moving or reshaping. The present thermal bilocation not only can fortify the performance of thermal camouflage, but also can stimulate the extension of illusion thermodynamics to further physical applications.

Design and realization of thermal camouflage with many-particle systems

An object can be found and identified by detecting its scattering signature of various physical fields, e.g., electromagnetics, acoustics, thermotics, etc. Similar to optical illusion, a thermal camouflage device can replace an expected object without altering the heat scattering pattern. Here, we propose and realize a two-dimensional metasurface, which is capable of controlling the heat flow and can be used as a device for misleading thermal sensors. The thermal metasurface is composed of conventional particles with many-body local-field effects. Our experiments and finite-element simulations have confirmed the thermal camouflage effect for both line and point heat sources, demonstrating the performance of thermal illusion. Also, we show that this many-particle structure can be extended to three-dimensional systems, which opens a door for designing applicable camouflage devices in the future.

Directed Thermal Diffusions through Metamaterial Source Illusion with Homogeneous Natural Media.

Owing to the utilization of transformation optics, many significant research and development achievements have expanded the applications of illusion devices into thermal fields. However, most of the current studies on relevant thermal illusions used to reshape the thermal fields are dependent of certain pre-designed geometric profiles with complicated conductivity configurations. In this paper, we propose a methodology for designing a new class of thermal source illusion devices for achieving directed thermal diffusions with natural homogeneous media. The employments of the space rotations in the linear transformation processes allow the directed thermal diffusions to be independent of the geometric profiles, and the utilization of natural homogeneous media improve the feasibility. Four schemes, with fewer types of homogeneous media filling the functional regions, are demonstrated in transient states. The expected performances are observed in each scheme. The related performance are analyzed by comparing the thermal distribution characteristics and the illusion effectiveness on the measured lines. The findings obtained in this paper see applications in the development of directed diffusions with minimal thermal loss, used in novel “multi-beam” thermal generation, thermal lenses, solar receivers, and waveguide.

Illusion Thermotics

"Fata Morgana" or "Mirage" phenomena have long been captivated as optical illusions, which actually relies on gradient-density air or vapor. Man-made optical illusions have witnessed significant progress by resorting to artificially structured metamaterials. Nevertheless, two long-standing challenges remain formidable: first, exotic parameters (negative or less than unity) become inevitable; second, the signature of original object is altered to that of a virtual counterpart. It is thus not able to address the holy grail of illusion per se, since a single virtual object still exposes the location. In this study, those problems are successfully addressed in a particular setup-illusion thermotics, which identically mimics the exterior thermal behavior of an equivalent reference and splits the interior original heat source into many virtual signatures. A general paradigm to design thermal illusion metadevices is proposed to manipulate thermal conduction, and empower robust simultaneous functions of moving, shaping, rotating, and splitting heat sources of arbitrary cross sections. The temperature profile inside the thermal metadevice can mislead the awareness of the real location, shape, size, and number of the actual heat sources. The present concept may trigger unprecedented development in other physical fields to realize multiple functionalized illusions in optics, electromagnetics, etc.

Periodic composites: quasi-uniform heat conduction, Janus thermal illusion, and illusion thermal diodes

Manipulating thermal conductivities at will plays a crucial role in controlling heat flow. By developing an effective medium theory including periodicity, here we experimentally show that nonuniform media can exhibit quasi-uniform heat conduction. This provides capabilities in proposing Janus thermal illusion and illusion thermal rectification. For the former, we study, via experiment and theory, a big periodic composite containing a small periodic composite with circular or elliptic particles. As a result, we reveal the Janus thermal illusion that describes the whole periodic system with both invisibility illusion along one direction and visibility illusion along the perpendicular direction, which is fundamentally different from the existing thermal illusions for misleading thermal detection. Further, the Janus illusion helps to design two different periodic systems that both work as thermal diodes but with nearly the same temperature distribution, heat fluxes and rectification ratios, thus being called illusion thermal diodes. Such thermal diodes differ from those extensively studied in the literature, and are useful for the areas that require both thermal rectification and thermal camouflage. This work not only opens a door for designing novel periodic composites in thermal camouflage and heat rectification, but also holds for achieving similar composites in other disciplines like electrostatics, magnetostatics, and particle dynamics.

Full control of heat transfer in single-particle structural materials

Thermal metamaterials have been applied to implement thermal phenomena, such as invisibility, illusion, and refraction. However, during the fabrication, they probably have complicated issues which are on account of the complicated structures. To get around this, here we put forward a single-particle structure. The theory helps to simplify the existing methods, which will undoubtedly contribute to the efficiency of fabrication. For clarity, we show the simulation and experimental results of thermal invisibility and illusion based on our proposed single-particle structural materials. Moreover, by tailoring the shape factor of the single particle appropriately, we can simultaneously realize thermal invisibility and illusion with only one device. The adjustable area fraction also indicates that these types of structural materials are highly adaptable. Such a single-particle device may have broad applications in misleading infrared detection. This work not only opens an avenue to design thermal materials based on single-particle structures but also holds for other physical fields like electrostatics, magnetostatics, and particle dynamics.

Two-dimensional thermal illusion device with arbitrary shape based on complementary media**Project supported by the National Natural Science Foundation of China (Grant No. 11504426) and the National Defense Foundation of China (Grant No. 1010502020202).

On the basis of transformation thermodynamics and compensation medium theory, we develop a method to design a two-dimensional thermal illusion device with arbitrary shape, and the general expression of thermal conductivity in the each region is obtained. Simulation results show that when an object is covered with the thermal illusion device, it will accurately perform the same temperature distribution signature as another object we have predetermined. Owing to the property of deceiving and interfering with the observer, the thermal illusion device can achieve generalized thermal stealth by using thermal metamaterials, which may have a potential application in military field.

Transformational fluctuation electrodynamics: application to thermal radiation illusion.

Thermal radiation is a universal property for all objects with temperatures above 0K. Every object with a specific shape and emissivity has its own thermal radiation signature; such signature allows the object to be detected and recognized which can be an undesirable situation. In this paper, we apply transformation optics theory to a thermal radiation problem to develop an electromagnetic illusion by controlling the thermal radiation signature of a given object. Starting from the fluctuation dissipation theorem where thermally fluctuating sources are related to the radiative losses, we demonstrate that it is possible for objects residing in two spaces, virtual and physical, to have the same thermal radiation signature if the complex permittivities and permeabilities satisfy the standard space transformations. We emphasize the invariance of the fluctuation electrodynamics physics under transformation, and show how this result allows the mimicking in thermal radiation. We illustrate the concept using the illusion paradigm in the two-dimensional space and a numerical calculation validates all predictions. Finally, we discuss limitations and extensions of the proposed technique.

Remote cooling by a novel thermal lens with anisotropic positive thermal conductivity

A novel thermal lens that can achieve a remote cooling effect is designed by transformation thermodynamics. The effective distance between the separate hot source and cold source is shortened by our shelled thermal lens without any negative thermal conductivity. Numerical simulations verify the performance of our thermal lens. Based on the effective medium theory, we also propose a practical way to realize our lens using two-layered isotropic thermal media that are both found in nature. The proposed thermal lens will have potential applications in remote temperature control and in creating other thermal illusions.

Design and research of three-dimensional thermal cloak with arbitrary shape based on the transformation thermodynamics

Based on the form-invariance of the thermal conduction equation different from wave equation, transformation thermodynamics has opened up a new area for the arbitrarily manipulating of heat fluxes at discretion by using thermal metamaterials. Moreover, it can help researchers to design different kinds of thermal devices with many unique properties that cannot be simply realized by natural materials, such as thermal cloaking, thermal concentrating, thermal rotating and thermal illusion. Among these devices, the conventional thermal cloak enabling heat fluxes to travel around the inner region, has attracted the most significant attention so far. At the present time, the studies of the thermal cloak mainly focus on two-dimensional space with arbitrary shape and three-dimensional space with regular shape, which appear to be far from enough to meet the engineering requirements. In this paper, we derive the general expression of the thermal conductivity for three-dimensional thermal cloak with arbitrary shape according to the transformation thermodynamics. In this paper, the thermal conductivity in the polar coordinate system is transformed into that in the Cartesian coordinate system by means of coordinate transformation. On the basis of the expression of the thermal conductivity, we adopt full-wave simulation by using the software COMSOL Multiphysics to analyze the cloaking performances of five designed thermal cloaks, i.e., spherical thermal cloak, ellipsoidal thermal cloak, three-dimensional conformal thermal cloak with arbitrary shapes, non-conformal thermal cloak with the sphere outside the ellipsoid, and three-dimensional non-conformal thermal cloak with arbitrary shapes. The results show that the heat fluxes travel around the protection area, and eventually return to their original paths. The temperature profile inside the thermal cloak keeps unchanged, and the temperature field outside the thermal cloak is not distorted, which proves that the cloak has a perfect thermal invisible effect. Both the conformal and non-conformal thermal cloak have perfect thermal protection and invisible function. In this paper, the transformation thermodynamics is extended from two-dimensional thermal cloak to three-dimensional thermal cloak with better universality. At the same time, this technology provides more flexibility in controlling heat flow and target temperature field, which will have potential applications in designing microchip, motor protection and target thermal stealth. It is believed that the method presented here can be applied to other branches of physics, such as acoustics, matter waves and elastic waves.

Transient thermal camouflage and heat signature control

Thermal metamaterials have been proposed to manipulate heat flux as a new way to cloak or camouflage objects in the infrared world. To date, however, thermal metamaterials only operate in the steady-state and exhibit detectable, transient heat signatures. In this letter, the theoretical basis for a thermal camouflaging technique with controlled transient diffusion is presented. This technique renders an object invisible in real time. More importantly, the thermal camouflaging device instantaneously generates a pre-designed heat signature and behaves as a perfect thermal illusion device. A metamaterial coating with homogeneous and isotropic thermal conductivity, density, and volumetric heat capacity was fabricated and very good camouflaging performance was achieved.

Thermal Magnifier and Minifier**Support by the National Natural Science Foundation of China under Grant No. 11222544, by the Fok Ying Tung Education Foundation under Grant No. 131008, by the Program for New Century Excellent Talents in University (NCET-12-0121), and by the Chinese National Key Basic Research Special Fund under Grant No. 2011CB922004

For thermal conduction cases, one can detect the size of an object explicitly by measuring the temperature distribution around it. If the temperature is the only signature we can obtain, we will give an incorrect judgment on the shape or size of the object by disturbing the distribution of it. According to this principle, in this article, we develop a transformation method and design a dual-functional thermal device, which can create a thermal illusion that the object inside it “seems” to appear bigger or smaller than its original size. This device can functionally switch among magnifier and minifier at will. The proposed device consists of two layers: the cloak and the complementary material. A thermal cloak can make the internal region thermally “invisible” while the complementary layer offsets this effect. The combination leads to the illusion of magnification and minification. As a result of finite element simulations, the performances of the illusions are confirmed.

Design, implementation, and extension of thermal invisibility cloaks

A thermal invisibility cloak, as inspired by optical invisibility cloaks, is a device which can steer the conductive heat flux around an isolated object without changing the ambient temperature distribution so that the object can be “invisible” to external thermal environment. While designs of thermal invisibility cloaks inherit previous theories from optical cloaks, the uniqueness of heat diffusion leads to more achievable implementations. Thermal invisibility cloaks, as well as the variations including thermal concentrator, rotator, and illusion devices, have potentials to be applied in thermal management, sensing and imaging applications. Here, we review the current knowledge of thermal invisibility cloaks in terms of their design and implementation in cloaking studies, and their extension as other functional devices.

A simple and flexible thermal illusion device and its experimental verification

A new kind of thermal illusion device is proposed, which requires one layer of homogeneous and isotropic thermal conducting materials. Unlike the transformation optics method, this illusion device is analytically designed by matching two distinct temperature profiles along a controlled boundary. We demonstrate that a good thermal conductor can be disguised as a poor one with the fabricated illusion device, and vice versa. And the simulation and experiment results agree well with each other. This kind of illusion device is very simple, flexible, and can be further improved for three dimensions. It also enjoys a variety of applications, such as military camouflage, heat flow manipulation, etc.

Illusion thermodynamics: A camouflage technique changing an object into another one with arbitrary cross section

The previously reported magical thermal devices, such as the thermal invisible cloak and the thermal concentrator, are generalized into one general case named here thermal illusion device. The thermal illusion device is displayed by the design of a thermal reshaper which can reshape an arbitrary thermal object into another one with arbitrary cross section. General expressions of the material parameters for the thermal reshaper are derived unambiguously to greatly facilitate the design of general thermal illusion device. We believe that this work will broaden the current research and pave a path to the thermal invisibility. Numerical simulations show good agreement with the analytical results of the thermal illusion device.

Anisotropic conductivity rotates heat fluxes in transient regimes

We present a finite element analysis of a diffusion problem involving a coated cylinder enabling the rotation of heat fluxes. The coating consists of a heterogeneous anisotropic conductivity deduced from a geometric transformation in the time dependent heat equation. In contrast to thermal cloak and concentrator, specific heat and density are not affected by the transformation in the rotator. Therein, thermal flux diffuses from region of lower temperature to higher temperature, leading to an apparent negative conductivity analogous to what was observed in transformed thermostatics. When a conducting object lies inside the rotator, it appears as if rotated by certain angle to an external observer, what can be seen as a thermal illusion. A structured rotator is finally proposed inspired by earlier designs of thermostatic and microwave rotators.

1P1-A04 触覚刺激提示部位への温度感覚転写現象の解明(触覚と力覚(3))

When the heat stimulation is presented to tip of the index finger and the vibratory stimulation is presented to tip of the middle finger, the middle finger is also perceived to be warm. This phenomenon is known as a kind of the thermal illusion called Thermal Referral. To reveal the mechanism of the Thermal Referral, we focused on tactile mechanoreceptors. We varied the frequencies of vibratory stimulation to stimulate different mechanoreceptors, and we evaluate the mechanoreceptors concerned with Thermal Referral.