French physicist Jean-Baptiste Joseph Fourier described the mathematical model of heat conduction in 1822—and his statement has been taught under the name Fourier's law for more than two centuries. The molecules of matter vibrate continuously in proportion to the temperature, and this movement spreads from the warmer part of the body to the cooler part. The substance itself does not move, but internal energy is transferred – in this process, the energy does not disappear, but equalizes, as is the case with temperature itself. The speed of propagation depends on the temperature difference between the two points and the area or diameter through which the heat can be transferred.
In recent decades, it has become known that this principle of thermal conduction does not work on the nanometer scale. Meanwhile, Cheng Caicai, a polymer physicist at the University of Massachusetts, and his colleagues wondered whether there might be an exception to the law in the more common macro world. Their assumption was that since transparent polymers and inorganic glass transmit light at a specific wavelength, heat also propagates in them as radiation. Radiant heat spreads in the form of electromagnetic waves, i.e. infrared rays, which is the heat flowing towards us from the sun.
The research began with a simple question: What if heat could be transferred in more than one way, as we have assumed so far?
– explained one of the authors of the research Steve Granek.
The researchers suspended strips of the material in a vacuum, eliminating the intermediary effect of air, and then began heating one end of the strips with laser pulses lasting for a fraction of a second. The result was measured in three ways: direct temperature measurement, color change of the heat-sensitive coating applied to the strips, and an infrared camera.
Heat spread faster than thermal conduction followed, so the scientists determined that radiation contributed to the energy transfer – this effect appeared immediately after the laser pulse, and subsequently diminished compared to the material's thermal conductivity.
This does not mean that Fourier's law is wrong, but it does not explain everything about heat transfer
Granik noted.
According to the researchers, internal thermal radiation appears in transparent materials because defects in the material's structure absorb and radiate heat – thus thermal radiation jumps from one point to another instead of transferring heat more slowly.
Since the result slightly expanded old knowledge, it could give engineers new strategies in cases where the problem of heat propagation in materials needs to be solved.
(Cover Photo: Olly Scarff/Getty Images)
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