Researchers find an exception to a 200-year-old scientific law governing heat transfer

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Cheng and Granik work in the lab. This image was taken using the infrared camera they used in their experiments. Colors measure temperatures. Notice that their skin is warm and their hair is cooler. Credit: UMass Amherst

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Cheng and Granik work in the lab. This image was taken using the infrared camera they used in their experiments. Colors measure temperatures. Notice that their skin is warm and their hair is cooler. Credit: UMass Amherst

A team of researchers led by the University of Massachusetts Amherst recently discovered an exception to the 200-year-old law, known as Fourier's Law, that governs how heat spreads through solid materials.

Although scientists have previously shown that there are exceptions to the law at the nanoscale, the research published in the journal Proceedings of the National Academy of Scienceswas the first to show that the law does not always apply at the macro level, and that pure electromagnetic radiation also operates in some common materials such as plastic and glass.

says Steve Granick, Robert K. “This research started with a simple question,” said Barrett Professor of Polymer Science and Engineering at the University of Massachusetts Amherst and lead author of the paper. “What if heat could be transmitted via another path, not just the path people have assumed?”

Radiant heat is the heat we feel from the sun; Its electromagnetic waves heat our skin when the sun shines. Diffusion, on the other hand, is how the tea cup will warm your hand after you pour yourself a new cup. For 200 years, scientists believed that diffusion explains how heat moves through solids. “But sometimes, creativity requires that you put down the textbook for a moment,” Granik says.

Granik, Shankar Ghosh of the Tata Institute of Fundamental Research and lead author Kaikai Cheng, a senior research fellow at the University of Massachusetts Amherst, believe an exception to Fourier's law can be found in transparent polymers and inorganic glasses. Heat radiates through both materials, but the team hypothesized that their transparency might also allow energy to radiate through the materials as well.

To test the hypothesis, they placed samples of materials in a vacuum chamber, which would eliminate the air responsible for the thermal distribution of heat. They then created a pulse of heat in one sample using a laser to heat a small area, and in the other sample, they heated one side while keeping the other side cool.

They then used a special infrared camera to watch the heat spread through their samples. By repeating the experiment several times, they kept finding anomalies that Fourier's law could not fully explain.

“No one has tried this before,” Cheng says. “There is something unexpected happening inside transparent polymers.”

It turns out that transparent materials allow energy to radiate internally, interacting with small structural defects, which then become secondary heat sources. These secondary heat sources continue to radiate heat through the material.

“It's not that Fourier's law is wrong,” Granik was quick to stress, “but it doesn't explain everything we see when it comes to heat transfer. Basic research like ours gives us an expanded understanding of how heat works, which will make it easier for us to understand how heat works.” Providing new strategies for engineers to design thermal circuits.”

more information:
Granik, Steve et al., Exceptions to Fourier's Law at the Macro-Level, Proceedings of the National Academy of Sciences (2024). doi: 10.1073/pnas.2320337121. doi.org/10.1073/pnas.2320337121

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