![]() ![]() ![]() So the resulting film will reflect visible light at specific wavelengths to get those aesthetically pleasing colors-in this case, light green and blue, although Shen said that one could adjust the cellulose nanocrystal suspension to get other colors like red. Once extracted, the nanocrystals are dispersed in water, and as the water evaporates, the nanocrystals self-assemble into a photonic crystal structure. One layer comprises cellulose nanocrystals, which can be extracted from renewable sources like wood, per Shen. The trick is to use two distinct layers with different functions. And being able to make large sheets of the material opens up applications in apparel and sportswear. Children would love wearing bandages that change color, providing a boon for pediatricians. The films would be ideal for making bandages that change color in response to pressure, letting medical professionals know if they are wrapping a wound too tightly-an important factor when treating conditions like venous ulcers, pressure ulcers, lymphoedema, and scarring. Scientists can make their own structural colored materials in the lab, but it can be challenging to scale up the process for commercial applications without sacrificing optical precision. So creating structural colors like those found in nature is an active area of materials research.įor example, last year, Massachusetts Institute of Technology scientists adapted a 19th-century holographic photography technique invented by physicist Gabriel Lippmann to develop chameleon-like films that change color when stretched. Alter the structure by changing the size of the tiles, and the crystals become sensitive to a different wavelength. Also known as photonic band gap materials, photonic crystals are "tunable," which means they are precisely ordered to block certain wavelengths of light while letting others through. Essentially, they form a diffraction grating, except photonic crystals only produce specific colors, or wavelengths, of light, while a diffraction grating will produce the entire spectrum, much like a prism. Advertisementįurther Reading MIT scientists create color-shifting films inspired by 19th-century holographyĪs previously reported, the bright iridescent colors in butterfly wings, soap bubbles, or beetle shells, for example, don't come from any pigment molecules but from how the wings are structured-a naturally occurring example of what physicists call photonic crystals. In nature, scales of chitin (a polysaccharide common to insects) are arranged like roof tiles. So color can be added without sacrificing the passive cooling efficiency. are colored, but it is structural color in the form of nanocrystals, not due to adding pigments or dyes. Pigments or dyes interfere with that since they absorb specific wavelengths of light and only reflect certain colors, thereby transforming energy from the light into heat. Most PDRC materials (paints, films, and so forth) are white, or have a mirrored finish, to achieve a broadband reflection of sunlight. ![]() Their cooling technology exploits that thermal transfer, with a twist. "We know there is spontaneous thermal transfer between objects with different temperatures," Qingchen Shen said at a press conference during the meeting. The surface emits its own heat into space without being absorbed by the air or atmosphere, thereby becoming several degrees cooler than the surrounding air without needing electrical energy. The technical term for this approach is passive daytime radiative cooling (PDRC), so named because it doesn't require an injection of energy into the system to disperse heat. They described their work at a recent meeting of the American Chemical Society. Scientists from the University of Cambridge have developed an innovative new plant-based film that gets cooler when exposed to sunlight, making it ideal for cooling buildings or cars in the future without needing any external power source. Hence, there is strong interest in coming up with eco-friendly alternatives. The downside to A/C is that the units gobble up energy and can emit greenhouse gases, contributing further to global warming. Summer is almost here, bringing higher temperatures and prompting many of us to crank up the air conditioning on particularly hot days. ![]()
0 Comments
Leave a Reply. |