Synopsis: With closeness comes greater warmth
When an “infinite” warm surface is separated from a cooler one by a vacuum gap, the rate of radiative heat transfer between the two shouldn’t depend on the size of the gap. According to theory, though, this picture doesn’t hold when the surfaces are sufficiently close. Now, a team of scientists reports in Physical Review Letters they have measured near-field radiative heat-transfer in a setup that allows them to directly compare their results with theoretical predictions.
Near-field effects occur because of evanescent electromagnetic fields—light that, because of total internal reflection, can’t penetrate a medium, and instead decays away from a surface. These fields dominate the radiative heat transfer process when two surfaces, held at different temperatures, are in close proximity. Though the effect has been detected in other experiments, the geometry of the surfaces made comparison with theoretical calculations difficult. In their paper, Rich Ottens and his colleagues at the University of Florida, Gainesville, focused on a more straightforward planar geometry. They measured the heat transfer between two parallel square sapphire plates, each about two inches on a side, but separated by only a few microns.
Ottens et al. see a pronounced increase in heat transfer as they reduce the gap between the plates that agrees well with theoretical predictions. In principle, near-field heat transfer could be used to control the temperature of an object without ever contacting it—an interesting possibility for cooling the sensitive mirrors in planned gravity wave detectors. – Jessica Thomas