Can you feel the heat? For a thermal imager, which measures infrared radiation, the heat we can feel is visible, like the heat of a traveler in an airport with a fever or the cold from a leaky window or door in winter. .
In an article published in Proceedings of the Royal Society A: Mathematical, physical and technical sciences, an international group of applied mathematicians and physicists, including Fernando Guevara Vasquez and Trent DeGiovanni of the University of Utah, report a theoretical way to imitate thermal objects or make objects invisible to thermal measurements. And it doesn’t require a Romulan camouflage device or Harry Potter’s invisibility cloak. The research is funded by the National Science Foundation.
The method allows fine tuning of heat transfer even in situations where the temperature changes over time, the researchers say. One application might be to isolate a part that generates heat in a circuit (eg, a power supply) to prevent it from interfering with heat-sensitive parts (eg, a thermal camera). Another application could be in industrial processes which require precise temperature control both in time and in space, for example by controlling the cooling of a material so that it crystallizes in a particular way.
Concealment or invisibility devices have long been part of fictional stories, but in recent years scientists and engineers have explored how to turn science fiction into reality. One approach, using metamaterials, bends the light so as to make an object invisible.
Just as our eyes see objects if they emit or reflect light, a thermal camera can see an object if they emit or reflect infrared radiation. In mathematical terms, an object could become invisible to a thermal imager if the heat sources placed around it could mimic heat transfer as if the object were not there.
New to the team’s approach is that it uses heat pumps rather than specially designed materials to hide objects. A simple household example of a heat pump is a refrigerator: to cool groceries, it pumps heat from the inside to the outside. Using heat pumps is much more flexible than using carefully designed materials, says Guevara. For example, researchers can make an object or source appear to be a completely different object or source. “So at least from a thermal measurement point of view,” says Guevara, “they can make an apple appear like an orange.”
The researchers did the mathematical work needed to show that with a ring of heat pumps around an object, it is possible to thermally hide an object or mimic the thermal signature of a different object.
The work remains moot, Guevara says, and the simulations assume a point source of heat that reflects or bends around the object – the thermal equivalent of a flashlight in a dark room.
The temperature of this probe source must be known in advance, a disadvantage of the work. However, the approach is within the reach of current technology by using small heat pumps called Peltier elements which transport heat by passing an electric current through a metal-to-metal junction. Peltier elements are already widely used in consumer and industrial applications.
The researchers envision that their work could be used to precisely control the temperature of an object in space and time, which has applications in the protection of electronic circuits. The results, the researchers say, could also be applied to precise drug delivery, because the mathematics of heat transfer and diffusion is similar to that of drug transfer and diffusion. And, they add, the mathematics of light behavior in diffuse media such as fog could also lead to applications in visual camouflage.
Besides Guevara and DeGiovanni, Maxence Cassier, CNRS researcher at the Fresnel Institute in Marseille, France and Sébastien Guenneau, CNRS researcher, UMI 2004 Abraham de Moivre-CNRS, Imperial College London, London, United Kingdom are co-authors of the study.
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