Some birds display incredibly exotic plumage, captivating the eyes of potential mates by flashing tantalizing feathers splashed with bright colors. What if we could call on the science behind these brilliant colors, to make a paint that never fades? Well, scientists are attempting just that.
In the majority of cases, colors in paints and dyes come from synthetic pigments. The color you see is a function of your visual system; objects don't intrinsically possess color. Rather, what you see is a reflection of light of a particular wavelength that corresponds to a visible color on the electromagnetic spectrum. Other wavelengths of light are absorbed instead of reflected; these therefore do not reach the cells found in the eye that act as color receptors, and so are not seen. Beautifully colored bird feathers, however, produce their color by a different mechanism; structural color. Herein lies the secret that scientists hope to exploit in industry.
Structural color is achieved when tiny (nano) structures amplify a particular wavelength of light. Cells found within feathers contain pores which are spaced in a particular manner; it is this spatial arrangement that causes the reflection of particular wavelengths of light, thus allowing us to see certain colors. For example, a type of bird called the cotinga has pores in its feathers that are arranged in such a way that only blue light is reflected, allowing us to see brilliantly blue feathers.
A team of researchers at Harvard, led by Professor Vinothan Manoharan, have been working hard on re-creating this structural color in man-made materials. In a paper published recently, the team filled microcapsules with a solution of particles suspended in water. The particles in solution were disordered, similar to that of the pores in the cotinga's feathers. As the capsule dries out, it begins to shrink, bringing the particles in closer proximity to one another. When the particles reach a certain distance from each other, a specific wavelength of light is reflected. But even more intriguingly, if you continue to dry out the capsule, bringing the particles even closer, a different wavelength is reflected. The varying distance between the particles therefore governs the wavelength of light reflected, and therefore the color seen.
Below is an example of a shrinking capsule changing color over time, so you can get your head around this tricky idea.
(Image: Jin-Gyu Park)
As mentioned, the color that we normally see from dyes and paints is due to the specific reflection and absorption of particular wavelengths of light. Over time, the energy from light which constantly bombards these molecules will cause them to deteriorate; this is why they fade. This new system, however, is hoped to provide a mechanism whereby this fading can be avoided. "We think it could be possible to create a full-color display that won't fade over time. The dream is that you could have a piece of flexible plastic that you can put graphics on in full color and read in bright sunlight" says Manoharan.
These capsules convey several advantages over conventional dyes currently on the market. Many synthetic dyes are toxic to humans, whereas almost any material can be used to create these capsules, so long as it is structured correctly. Therefore, non-toxic substances can be selected. Other dyes come from natural sources, such as the red pigment carmine, which is extracted from an insect called a cochineal. This is an exceedingly labor intensive process and involves harvesting large numbers of insects, which could be avoided using this methodology.
Although the research is still in its infancy and at the experimental stage, the group hope to be able to eventually commercialize this technique, which could see the advent of exciting new products on the market.