Keeping cool without losing your shirt

Keeping cool without losing your shirt






Keeping cool without losing your shirt

New Delhi : Researchers have developed a cloth that reflects sunlight and also allows heat radiating from a person's body to escape - shedding major forms of heat from an individual's body. While humans have been wearing furs and clothes to keep warm for all of history, the development of "cooler" clothing for hot summer days has remained challenging. This is because, at normal skin temperatures of 34 degree Celsius, the human body emits mid-infrared (IR) radiation (in form of heat) in the wavelength range that partially overlaps with that of the visible light spectrum.

Both infrared, which is invisible, and visible light popularly known as different colours, are part of the electromagnetic spectrum – which comprise of different forms of light waves from the invisible gamma ray, x-ray and ultraviolet on the one end of the spectrum and infrared and radio waves, on the other end.
In between is what enables us to see and distinguish colours – which can be scientifically termed as ‘visible light’. Therefore, cloth that blocks visible light often traps in body heat. Here, Po-Chun Hsu, Yi Cui and colleagues explored an existing nanoporous (having microscopic pores) polyethylene (nanoPE), evaluating whether it would allow IR-radiation to pass through, and thus make a likely cool clothing candidate.

NanoPE has interconnected pores that are 50 to 1,000 nanometers in diameter, comparable in size to the wavelength of visible light, and thus capable of scattering it, or reflecting it back. Yet, at these sizes, the pores are much smaller than the wavelength of infrared light, meaning nanoPE is still highly transparent to IR. The team tested nanoPE and cotton, finding the former to allow 96 per cent of IR to pass through, compared to its cotton counterpart, which only permitted the passage of 1 point 5 per cent of infrared waves. Regular polyethylene lets similar amounts of IR through, but the nano PE significantly outperforms this traditional cloth in reflecting visible light, at 99 per cent opacity compared to 20 per cent in case of cotton.

In experiments with a device that mimics the heat output of human skin, the authors found that nanoPE only heats the simulated skin temperature by 0 point 8 degree C, compared with a heat boost of 3 point 5 degree C for cotton and 2 point 9°C for commercially available polyethylene fabrics.
What's more, whereas normal polyethylene does not wick away moisture well, here, the researchers were able to enhance fluid wicking of the nano PE using a microneedle punching technique and by coating the material with a water-repellent agent. In a related Perspective, Svetlana Boriskina highlights how this new material could reduce energy costs associated with keeping body temperatures cool (ie, air conditioning costs), and it could even be used to address the thermal management of tents, buildings, and vehicles.

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