03 Apr 2018
World innovation news
Materials & Manufacturing
How Nature Uses Physics to Make the Color Blue
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A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption . Most color pigments are not created in our body, they come from consuming a variety of foods in our diets. We know this information by studying flamingoes. They are born gray, but turn pink as a result of pigments called carotenoids, which produce the majority of bright yellow, orange or reddish colors, found in crustaceans and algae they ingest. This effect does not exist for the color blue, so how do monkeys, birds or butterflies get their blue color?
Biological pigments are mainly organic compounds that selectively absorb incoming light and reflect the remaining wavelengths appearing as color. The color blue has shorter wavelengths, 492 – 455 nm, compared to green and red that have wavelengths of 577 – 492 nm and 780 – 622 nm, respectively. Most biological pigments absorb blue light because it provides sufficient energy to raise an orbital electron to an excited state .
Due to the rarity of blue pigments, nature often uses a pure physical phenomena to create blue called structural color, the physics of light at the nanoscale. When light hits the cell wall or the exoskeleton of an organism, it interacts with the micro- and nanostructures of their body. Reflection, refraction, interference, diffraction and scattering can become sources of structural colors and can be enhanced by combining these optical phenomena . As an example, the feathers of the common blue jay aren’t really blue, they’re gray. The feathers refract light, appearing blue to the human eye.
The morpho butterfly is one of the most studied insects in understanding structural color. The wings of a morpho butterfly are some of the most beautiful structures in nature, and yet they contain no blue pigment.
Nipam Patel’s lab at UC Berkeley is studying how morpho butterflies form the special structures, the scales that cover their wings while still inside the pupae. Each scale is like a pixel, tiny pieces of tile in a larger mosaic, set in layers of overlapping rows. Ridges on the scale’s surface are a key component that affects how the wing spreads or refracts light, similar to a prism. When light hits the ridges, a phenomenon called constructive interference comes into play, which is essentially when the diffracted light waves are intensified and reflected. The spacing within the ridges, which look like little “Christmas trees”, perfectly reinforces specific wavelengths while canceling out others. This is how the eye perceives that shimmering blue. Scientists are still unsure why, but vertebrates and plants rarely produce blue as a pigment.
Why are people studying structural color? To develop photonic components for creating, manipulating or detecting light. This includes laser diodes, light-emitting diodes, solar and photovoltaic cells, displays and optical amplifiers. Other examples would be devices that modulate a beam of light, and combine and separate beams of light of different wavelengths.
Marie-Anne Valiquette obtained a Bachelor's degree in Mechanical Engineering at the École de technologie supérieure (ÉTS) in Montreal. She lives in Silicon Valley, California where she studies artificial intelligence through online platforms like Udacity and deeplearning.ai.
Program : Mechanical Engineering