Letting the heat leak out
(appeared in Sep 2016)

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A body cover that helps us stay cool in warm weather would reduce the cost of climate control, says S.Ananthanarayanan.

The principal purpose of clothing has been protection of the wearer from the cold. Better personal insulation could thus help most of the western world save on winter heating. But another large cost that the modern world faces, and one which would grow with global warming, is keeping the environment cool in warm weather. Clothing that does the opposite of insulating our bodies may be a way to economise on how cool we need the surroundings to be.

Po-Chun Hsu, Alex Y. Song, Peter B. Catrysse, Chong Liu, Yucan Peng, Jin Xie, Shanhui Fan and Yi Cui, from the departments of material science and electrical engineering at the University of Stanford and at the Linear Accelerator facility at Stanford, report in the AAAS journal, Science, that they have developed a material that does just that.

The way woolen or other fabric help us keep the body heat in is by being poor conductors of heat and impervious to winds. Insulating materials work by trapping pockets of air whose dimensions are nearly the same or less than the average distance that one molecule in the air moves before it encounters another. The thinner the fibres of the material and the closer the fibres are packed, the better the insulation. The reason why we need to conserve body heat in the winter is that body is much warmer than the surroundings and would rapidly lose heat if exposed. But in warm weather, there is much heat coming in by radiation from warm surroundings and the body needs to lose heat. Any clothes we wear act as a blanket and create a warm layer around the body. And if we need to lose more heat because of exertion, the warmth can be oppressive.

When we need to cool rapidly, because of exertion or because of a warm environment, the sweat glands get active and cover the body with sweat. The sweat carries out some of the heat, and when it evaporates, it cools the body surface and draws out more heat. But if the body is covered by clothes, and of course, it usually is, sweat would collect and discomfort would multiply. There is a way to treat fabrics so that they draw the sweat out to the exterior, when it could dry and cause some cooling, but this treatment gets active only when there is sweat, which is to say, when the body is pretty heated up already.

Yet another warm cover that we have is the atmosphere itself. The sun, which is very hot, some 6,000C, radiates mainly in visible light. The shortest light waves, towards the blue end of the spectrum, get scattered by the tiniest particles in the air but most of the energy, towards the red end of the spectrum, reaches ground level. The longer waves, the warm, infrared, do not get scattered by particles in the air, but they get absorbed by large molecules, like CO2 or methane, in the air, and warm up the atmosphere. These molecules also absorb the heat that the earth itself radiates and keeps the earth from cooling down at night.

The Stanford group took the lead from the atmosphere to create a fabric that would allow heat waves to pass through and still block light of the visible kind, so that the fabric would perform the social role of clothing. The reason that light at the blue end of the spectrum is scattered by the atmosphere, which gives the sky its blue colour, is that the air contains particles that are of the dimensions, around 500 nanometers, of the wavelength of blue light. Infrared waves, at more than 1000 nanometers, are much longer than these particles and do not get affected.

Nano polyethylene

The Stanford group examined an existing, normally transparent polyethylene fabric, but with the provision of nano-pores from 50 to 1000 nanometers in diameter. As the pores are of sizes comparable with the wavelength of visible light, visible light gets strongly scattered and the material is no longer transparent. The pores, however, are much smaller than infrared heat waves, which are able to freely pass through the material. The material, nanoPE, is found to allow 96% of the heat waves to pass through, against only 1.5% in the case of cotton fabric, which we usually wear in the summer season. Normal polyethylene is also as good at allowing heat to escape, but it allows 80% of visible light to pass through. NanoPE, on the other hand, is 99% opaque. In trials conducted on a surface that mimics the temperature of human skin, it was found that a drape of nanoPE causes a rise in temperature of only 0.8C, as against 3.5C for cotton and 2.9C for commercially available, fibrous PE textile.

Cotton clothing, despite the greater warming, is preferred to normal polyethylene because it soaks up any sweat. Polyethylene garments, in contrast, allow sweat to collect and can be quite uncomfortable in humid weather. One solution has been the use of wicking treatment, of a water repellent material that prevents the fabric in contact with the skin from getting soaked, along with a capillary structure that draws moisture to the outer side. This treatment has been found to help cotton fabric too. Use of this treatment could similarly increase the comfort of clothing made of nanoPE whose porous construction also permits permeation of air.

Sending the heat out to space

Considering that outer space, at temperatures near absolute zero (-273C), it should be simple for things on the earth or the earth itself to cool. Why this is not so is that the atmosphere keeps all infrared radiation from escaping from the earth. Warm things that radiate hence end up warming their surroundings and getting warmed in turn.

Except that there is a small window of frequencies of the infrared to which the atmosphere is truly transparent. Linxiao Zhu, Aswath P. Raman, Marc Abou Anoma, Eden Rephaeli and Shanhui Fan, also of Stanford University, had reported in 2014 that they had developed a surface, a photonic radiative cooler, that could radiate heat at the very frequency that has free passage through the atmosphere. An object covered with this surface could absorb heat from the sun and still grow cooler by radiating its own heat straight out to space.

Perfect transparence

Even transparent surfaces, like glass, do not allow all light that falls on them to pass through they reflect part of the light. The reason for reflection is that the electric and magnetic properties of media, which determine the speed of light waves, change when light passes from one medium to another.

A group at Karlsruhe Institute of Technology, in Karlsruhe, Germany and the Centre for Solar Energy and Hydrogen Research, Stuttgart, has recently studied the structure of natural surfaces that are able to do better - the cornea of the eye of the moth, a night-time creature that needs to capture all the light that it receives, and the surface of the rose petal, which needs to make the best use of light for photosynthesis.

The surfaces have microscopic, conical protuberances, of the dimensions of the wavelength of light, which affect the light waves and smooth the transition from one medium to the other. There is now a method of transferring the pattern to a film to lay on solar cells, and ensure that all the light that falls on the cells is made use of for generating electricity and not reflected and lost.

Coating the inner surface of nanoPE fabric with this pattern would make sure that heat radiation from the body actually goes out and is not reflected back in.

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