The view inside
the Ultra High Vacuum Scanning Thermal Microscope, which was used
to measure
temperature fluxes at the nanoscale. Image credit: Joseph Xu
(December 10, 2015)
When heat travels between two objects that aren't touching, it flows
differently at the smallest scales—distances on the order of the diameter of
DNA, or 1/50,000 of a human hair.
While researchers have been aware of this for decades, they
haven't understood the process. Heat flow often needs to be prevented or
harnessed and the lack of an accurate way to predict it represents a bottleneck
in nanotechnology development.
Now, in a unique ultra-low vibration lab at the University
of Michigan, engineers have measured how heat radiates from one surface to
another in a vacuum at distances down to 2 nanometers.
While the thermal energy still flows from the warmer place
to the colder one, the researchers found it does so 10,000 times faster than it
would at the scale of, say, a bonfire and a pair of chilly hands.
"Faster" here refers to the speed at which the temperature of one
sample changes the temperature of the other—and not the speed at which the heat
itself travels. Heat is a form of electromagnetic radiation, so it moves at the
speed of light. What's different at the nanoscale is the efficiency of the
process.
The view inside the Ultra High Vacuum Scanning Thermal Microscope, which was used to
measure temperature fluxes at the nanoscale. Image credit: Joseph Xu
"We've shown, for the first time, the dramatic
enhancements of radiative heat fluxes in the extreme near-field," said
Pramod Reddy, associate professor of mechanical engineering and materials
science and engineering. "Our experiments and calculations imply that heat
flows several orders of magnitude faster in these ultra small gaps."