A team of researchers from Germany and France reporting in the Physical Review Letters has developed and successfully tested a form of invisibility cloak that can guide the flow of heat around objects, making them thermally invisible.
Light and sound can be passed around objects by means of special metamaterials. The scientists now succeeded in demonstrating that the same materials can also be used to specifically influence the propagation of heat. They found that a structured plate made of copper and silicon can conduct heat around a central area without the edge being affected.
“For the thermal invisibility cloak, both materials have to be arranged smartly,” said study lead author Dr Robert Schittny from the Karlsruhe Institute of Technology.
“Copper is a good heat conductor, while the silicon material used, called PDMS, is a bad conductor. By providing a thin copper plate with annular silicon structures, we produce a material that conducts heat in various directions at variable speeds. In this way, the time needed for passing around a hidden object can be compensated.”
If a simple, solid metal plate is heated at the left edge, heat migrates uniformly to the right side. The temperature of the plate decreases from the left to the right. Exactly the same behavior is exhibited by the new metamaterial consisting of copper and silicon outside of the annular structure. No heat penetrates inside. Outside, there is no indication of what happens inside.
“These results impressingly reveal that transformation optics methods can be transferred to the highly different area of thermodynamics,” explained senior author Dr Martin Wegener, also from the Karlsruhe Institute of Technology. “Here, the first three-dimensional invisibility cloak for visible light was developed.”
“While optics and acoustics are based on the propagation of waves, heat is a measure of the unordered movement of atoms. Still, basic mathematical descriptions can be used to calculate the structures having the effect of an invisibility cloak. With the methods of so-called transformation optics, a distortion of the describing coordinate system is calculated. Arithmetically speaking, an extended object disappears in an infinitely small point. This virtual distortion can be mapped to a real metamaterial structure that passes incident light around the object to be hidden, as if it was not even existing.”
“I hope that our work will be the basis of many further developments in the field of thermodynamic metamaterials. Thermal invisibility cloaks are a rather new field in fundamental research. In the long term, they might be applied in areas needing effective heat management, such as in microchips, electric components, or machines.”
Bibliographic information: Robert Schittny et al. 2013. Experiments on Transformation Thermodynamics: Molding the Flow of Heat. Phys. Rev. Lett., vol. 110, no. 19; doi: 10.1103/PhysRevLett.110.195901