The Structure of Color

When I was young I learned that the colors I saw in the world were due to materials absorbing certain wavelengths of light and reflecting others. Leaves appear green because the chlorophyll in plant cells absorbs blue and red light and reflects green. While this holds true for most natural materials, it turns out that nature has more than one way of making color. Many of the most striking colors found in nature often occur through the development of structural coloration. For instance, the absurdly bright colors of butterfly wings, beetle scales, day gecko skin, bird feathers and even certain berries all come from structural color.

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Bright green gecko. Image credit: Samantha Levang via Flickr creative commons.

In animals and plants, structural coloration manifests as proteins that physically change the light that is reflected. The proteins form repeating structures that act as a filter, so when light enters the material the photons are separated from each other at a specific distance. This causes interference, changing the color we perceive. These filters can be formed from thin films of two different proteins in close contact, from stacked layers of proteins, or from three-dimensional arrangements of proteins called photonic crystals. The most important part of these arrangements is that they occur on the nanoscale. Since light travels in wavelengths that are also on the nanoscale, the proteins and photons can physically, rather than chemically, interact. The protein nanostructures change how the photon moves along its path, creating the color we see.

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The brightest blue that you ever did see. Image credit: Renee via Flickr creative commons.

A particularly striking example of the structural color phenomena can be found in the magnificent morpho butterfly’s wings. All butterfly wings are made of tiny scales, but morpho butterflies’ scales have microscopic ridges that preferentially reflect blue light. This extremely bright blue had long been thought to be the most intense color in the natural world. However, the discovery of Pollia condensata, an african plant with shockingly blue berries, supersedes even the lustrous butterfly. While both derive their brilliance from structural color, the internal configurations are quite different. The berry gets its brightness from cells containing layers of cellulose that are stacked the same distance apart as the wavelength of blue light. Thus the blue light gets reflected, while other wavelengths pass through. But what makes this berry really shine is that each cellulose layer is twisted slightly away from the one above it, so that some photons are reflected as circularly polarized light. What’s more, different cells have either a right or left handed corkscrew of cellulose, so the polarized light spins in both directions as it leaves the surface of the berry. While humans cannot sense circularly polarized light, scientists theorize that this helps birds and insects see the berry, increasing it’s chances of dispersion.

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Circularly polarized light travels in a corkscrew pattern. Image credit: Blake Bolinger via Flickr creative commons.

So why does structural coloration matter to you and me? Well, if scientists can understand how to change and control structural color, the way that chameleons or cuttlefish can, they can be used to create new display technologies. Imagine a hybrid of the liquid crystalline (LCD) and E-ink displays we have today; such screens could provide brilliant colors and use less energy at the same time. Additionally understanding how structural colors work can enable us to better control and manipulate light for communications networks. Materials could be manufactured that transmit light much further and more efficiently than current fiber optic cables. Future applications for structural colors could include cosmetics, paints, and other decorative finishes. Structural colors don’t fade like pigments do, and methods using pigments and dyes to color products can be environmentally unfriendly. Unfortunately, because manufacturing structural colors requires nanoscale level control of materials, such technologies have not yet been widely developed.

The natural world contains some truly astonishing phenomena. A number of the beautiful colors plants and animals produce are due to nanoscopic structures that we have just begun to comprehend. By investigating how nature created these colors, we can better understand how to improve materials in our own lives. Maybe someday we will be able to see a butterfly on a screen, as bright and beautiful as the one in real life.


Citations:

Fu, Y., Tippets, C. A., Donev, E. U. & Lopez, R. Structural colors: from natural to artificial systems. WIREs Nanomed Nanobiotechnol 2016, 8:758–775. doi: 10.1002/wnan.1396

 

About the Author

image2Hailing from the deserts of Arizona, Mackenzie Carter is an enthusiastic masters student in the College of Veterinary Medicine. She is currently studying tissue engineering to model disease states in bone, namely panosteitis in canines. Mackenzie loves hands on projects, from ceramics to solar powered robots. In her free time she explores her passions: cephalopods, tea, and swing dancing. You can connect with Mackenzie via email at mackenziecarter@uga.edu.