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THE SAME SCIENTIFIC principle that explains a blue sky might bring new fabrics that keep us cool. Researchers have recently engineered polyethylenes that are measurably better than cotton or “cool” synthetics in allowing body heat to radiate outward, yet retain opacity.
”Nanoporous Fabrics Could Keep You Cool,” by Svetlana V. Boriskina, is in the September 2, 2016, issue of Science, the weekly magazine of the American Association for the Advancement of Science. This article accompanies the complete research paper, “Radiative Human Body Cooling by Nanoporous Polyethylene Textile,” by Stanford University’s Po-Chun Hsu et al., in the same issue. Abstracts of the summary and of the research paper are freely available. Here are some tidbits.
The idea was to devise a fabric mesh large enough to optimize the escape of heat, but not so large as to be transparent.
As noted by Svetlana V. Boriskina in Science, “The human body is an almost perfect emitter of thermal radiation in the mid-infrared (IR) range. However, traditional fabrics block mid-IR waves by partially reflecting them and partially absorbing the thermal energy.” This is doubly bad: The fabrics get warm, yet bounce back heat onto the person.
The trick is to devise a fabric with pores optimized to let mid-IR wavelengths through, yet be opaque in the visible range.
Blue skies suggest a solution for this new fabric: The physics phenomenon of Mei scattering affects the visible spectrum’s shorter wavelengths–the blue ones–more than other colors, and thus we perceive the sky as blue.
Similarly, suppose a fabric has pore size comparable to the wavelengths of the visible spectrum– and much smaller than the mid-IR range. Then the visible light is scattered, and we perceive the fabric as opaque. Yet it’s transparent to the mid-IR range and lets these thermal emissions pass through.
In particular, the wavelengths of visible light are around 400 to 700 nanometers. By contrast, mid-IR radiation wavelengths are much larger, 7 to 14 microns (i.e., 7000 to 14,000 nanometers). [Thanks, H. Nelson, for this correction.]
As noted here in ”Nano Bright,” the world of nanotechnology is an amazing one. One nanometer is a billionth of a meter: a marble compared with the size of Earth. The technical challenge is working with such extremely thin fibers.
Hsu and Stanford colleagues produced fine strands for a textile promoting effective radiative cooling while still having sufficient air permeability, water-wicking rate and mechanical strength for wearability. In wrapped test samples, the resulting textiles exhibited temperatures almost 3 Celsius degrees cooler than cotton counterparts.
Nature is ahead of us in keeping nano-cool. The skin hairs of a Saharan silver ant are sufficiently fine to scatter and reflect sunlight, yet they’re transparent to IR wavelengths and thus they shed heat. Research has shown that, sans this protection, the ant’s body temperature is raised by a crucial couple of degrees.
To show the potential of nanoporous fabrics, Science author Boriskina observes that such clothes “could allow air conditioning setpoints to be higher than usual while maintaining the same level of personal thermal comfort. Depending on climate, a 1 to 4 Celsius degree increase in the setpoint temperature could save up to 45 percent of the energy required for the building’s cooling.”
What’s more, such revolutionary textiles wouldn’t be solely for clothing. Tents, buildings and vehicles could be designed with considerable savings in the management of thermal energy. ds
© Dennis Simanaitis, SimanaitisSays.com, 2016[