(C) Daily Kos This story was originally published by Daily Kos and is unaltered. . . . . . . . . . . Butterflies inspire materials that stay cool in sunlight despite their deep, vibrant colors [1] ['This Content Is Not Subject To Review Daily Kos Staff Prior To Publication.'] Date: 2023-08-05 You know how uncomfortable it can be to wear a dark-colored shirt on a hot, sunny day. Traditionally, dark-colored objects appear dark because they absorb a lot of light, and the energy from that light gets translated into molecules moving faster and vibrating and stretching more, which means higher temperature. The same thing is true for cars and buildings. Darker colors mean more heating... The heating effect of color with buildings and cars. Blue and white segments of a building in conventional (a) and infrared (b) photographs. Blue and white cars in a conventional (c) and infrared (d) photographs. Temperature scale at right. ...and when the weather is warm, that means that more energy needs to be devoted to keeping the inside of the building or car cool. A car, at least, moves pretty fast and can shed some heat that way, but not so for a building. When there’s no breeze, a warm wall won’t be in contact with cooler air; instead, a sort of heat bubble called a boundary layer will form, so the warm wall will be next to warm air, and cooling will be difficult. You can see below that when a warm object is met with a breeze of cooler air (on the left side), the air right next to the object is somewhat cooler, but where there’s little or no breeze (on the right side), the air right next to it is just as warm as the object, so not a lot of cooling is going to happen. The black arrows indicate air velocity and direction Before all this global heating, we had the luxury of not worrying about this very much. Just crank up the AC! But we cannot be that cavalier anymore. So does this mean we have to have all-white or all-mirrored buildings now? Well, no, not if we can use structural color — color produced without any absorption of light. A great natural example is the blue morpho butterfly, which appears strikingly blue even though its wings contain no blue pigment. We’ve learned more and more recently about how we can design this kind of structural color into materials, and even more recently our ability to fabricate nanostructures has been catching up to the theoretical knowledge. A team of researchers from Shenzhen University saw an opportunity here. They synthesized theory and practice to develop vibrantly colored materials (wait till you see them) that actually stay a few degrees cooler than the ambient temperature! They lay it out for us in an August 3 report in the journal Optica. OK, let’s get right down to what they’ve produced, because it’s pretty awesome. These are flexible films that appear as a variety of vibrant colors from a wide range of viewing angles, and they don’t rely on pigment at all for their color, but rather inexpensive materials arranged into very thin layers in just the right way. Here we’re going to compare a couple of things. On top we have a panel simply painted blue, left out on a car hood on a summer day. Then on the bottom we have an array of colored panels designed by our researchers, out there on the hood of a car parked next to the first one. Not only do the designed panels have way more-vibrant colors, but they stay a full 35°C (95°F) cooler than the painted panel! It’s absolutely flabbergasting. Name your color! We’ve got a panel for you How could they achieve something like that? They credit the butterfly for the crux of the idea, but they should get a lot of credit, too, because the calculations and nanofabrication they did were really impressive. The basic trick behind the colorful butterfly wing comes down to alternating layers of very thin materials. The thickness of the materials has to be similar to the wavelength of the color of light we want to produce. For blue, that’s on the order of 470 nanometers. That’s pretty thin, considering a piece of paper is about 100,000 nanometers thick! The butterfly wing uses alternating layers of the biopolymer chitin (pronouned KY-tin) and air: Scanning electron microscope images of (a) an oblique view and (b) a cross-section of a ground scale of the male butterfly Morpho didius The layer thickness is reasonably uniform, and that’s important for reflecting a single color. Here’s an illustration of that for just one layer, to keep things simple. We show two beams of light of the same wavelength coming in. They’re colored differently in the figure so you can tell them apart, but of course they’d represent the same color in real life. The two horizontal lines are the top and bottom of a layer of chitin. Chitin is partially reflective, so some of each beam gets reflected off the top and bottom of our layer, and some goes all the way through. We try to draw the figure so that a wave stays at the same place in its cycle when it bounces off a surface. When we have a good match between wavelength and layer thickness, all the outgoing waves line up nicely regardless of their incoming position, and whatever color this wavelength is gets magnified. All the wavecrests line up, and we get constructive interference: Constructive interference, when the wavelength of the incoming light is compatible with the layer thickness Now let’s make the wavelength shorter so we don’t have a good match with the chitin layer. Now we wind up with a hodgepodge of crests and troughs, and whatever color this wavelength is gets diminished by destructive interference: Destructive interference, when the wavelength of the incoming light is not compatible with the layer thickness Of course, with all those layers, it’s not as simple as these diagrams. Conveying even a portion of the reality turns into something like this: And there are way more layers, beams, and wavelengths than this! That’s why we leave it to these smart engineers to do all the necessary calculations, and then figure out how to fabricate the layers. They used silicon dioxide and titanium dioxide for the multilayer part, so cheap and easy there. Underneath that was a layer of frosted glass to disperse any light that makes it through all the layers, then at the bottom a thin film of silver as a total reflector. But the authors are right to say you can just replace that with aluminum. So we’re talking about inexpensive materials here. The end result is this flexible film that shows its color out pretty much regardless of the viewing angle: The authors calculated what this material would mean for a building with 100 square meters of absorptive area in a typical year’s exposure to sunlight in Shenzhen, China. A roof with pigmented shingles would swallow up 122 gigajoules per year, but a roof with this material would actually have a net LOSS of 16 gigajoules per year. When you take radiative heat loss into account, this material actually achieves a small amount of passive cooling. Cooling! I am shaking my head here. So look, I know we could achieve all this with bland white-colored structures and roofs and cars and clothing and land surfaces, but why would we want to do that? Y’all can pick your colors now! We can make it work, and this is yet another reason why nature offers us far too much instruction to let numerous species go extinct. We have a laboratory all over the planet that’s been hard at work for billions of years optimizing all kinds of practical things. We can’t be dumb enough to let all of that slip away, can we? [END] --- [1] Url: https://dailykos.com/stories/2023/8/5/2185352/-Butterflies-inspire-materials-that-stay-cool-in-sunlight-despite-their-deep-vibrant-colors Published and (C) by Daily Kos Content appears here under this condition or license: Site content may be used for any purpose without permission unless otherwise specified. via Magical.Fish Gopher News Feeds: gopher://magical.fish/1/feeds/news/dailykos/