Structural color. The butterfly wings iridescent secret

What is color?

There is no doubt. Something we can learn from nature is color and how to use it. Life on earth displays itself with a proud myriad of colors, shining and shimmering with outstanding power in some exceptional organisms. Disclaimer: I’m ignoring camouflage in this post…

Many of these nature jewels, like birds and butterflies with greens and blues that delight everyone’s eyes, hide a secret called structural color.

But, what is that we call color in the first place? Usually, color is perceived like the result of the interaction of light with a pigment in the object we are looking at, where only some colors (or wavelengths, the same for our purpose) of the beam are absorbed by the object, while the others are reflected and end reaching our eyes. Pigments absorb these colors by means of electronic transitions that match the energy of the colors absorbed. This is a property of the material’s nature itself. Therefore, if you shape and change the material’s body, the color doesn’t change, right? I don’t want to get too deep in physics, but we need to understand this to discern the differences regarding structural color.

Well, here is where the thing gets a little tricky. If there’s a structure on the material the size of the wavelenghts that constitute the visible light spectrum (this is less than a micron, between 380 and 750 nanometers) a phenomena called wave-interference will take place. This interference can be constructive, resulting in a reinforced color, or destructive, resulting in the partial or complete extinction of that color. You see where we’re going?

peacock feather
Peacock feather. Another example of structural color in nature.

Nanometric frameworks on scales, feathers and carapaces lead to iridescence and reinforcement of some colors, while others get destroyed in the process. This way, nature can use a translucent chitinous material and, by means of these nanometric structure, create a delightful iridescent walking beauty.

My favorite time, the exampling time:

The blue paradox

Morpho butterflies, a genus of over 29 different species, are one of those representative cases very well studied. It’s being a while since the interest in this kind of structures in butterflies rupture (since 1999 more or less), but not everything is said or discovered either; not at all.

And because a picture is worth a thousand words, here is the nanometric structure of the small scales that cover the blue reflecting wings of this insects:

wing structure.png
Wing hierarchic scheme. Note the two different types of scales in b). The basal one presents the tree-shaped nanometric structures we see in the lower images. This structure is called lamellar ridge array.

This Christmas tree shaped structures, which are the key of the blue iridescence, are made of chitin; a biopolymer we have talked about in this post, which has a refractive index of 1.56. This is important considering this network can be regarded as a 1D photonic crystal, so a high refractive material is needed. But there’ll be time to talk about photonic crystal in the future, so let’s focus on explaining how light gets “filtrated” by this diffraction grating-like structure.

When light beam hits the first perpendicular branch surface of one of these trees, it does so with an angle, as we see in the schematic figure down below. Most of the light pass through the translucent material, but some gets reflected. As light goes through every layer, and due to the constructive and destructive interferences we talked about, only the components of the light with the right wavelenght (color) endure and get reinforced.

interference diagram morpho.png
Constructive and destructive interference induced by the peculiar structure on the Morpho butterfly’s wings.

 

soap-bubble.jpg
Thin film iridescence in soap bubble.

The length of the path that the light travels between the surface and the bottom of every branch in the tree (lamellae) determines which color gets reinforced and which destroyed. But, do you remember I said the light beam reaches the surface with an angle? That angle changes the distance the light travels between layers, right? So color changes with the viewing angle…that is IRIDESCENCE! The same phenomena you see when a small layer of oil, gasoline, soap…spreads over a surface of water. This very thin film (click the link!) acts just like the small branches in our structure, but only with one layer (hence with a very week intensity), and the angle we look at the surface determines the color we perceive.

angle reliance morpho
Color-angle relation in morpho butterfly’s wings. Image taken from [5]
Then, why Morpho butterflies’ wings are just blue colored and not just iridescent?! Comprehending this have taken me some investigation time and effort. And by the way, this is also why this structure brings so much attention: the color-selective nature in these wings. The truth is in fact, that the color change with the angle: But just slightly, as you can see in the figure aside.

 

But while it is clear that blue absolutely prevails in this structure,  I’m afraid that there is no simple and quick answer for completely describing the nature of this characteristic. But I’ll try to make it simple and painless: It seems it’s all about a combination of the random distribution of the ridges (tops of the Christmas trees) at different heights, and the effect of a second kind of scales, the cover scales, which spread the blue light in all directions, and not only the one with the correct angle. The green and reddish colors are also absorbed, somewhat at least, by the pigmentation of the scales. This increases also the blue hue.

There are even more light-effects produced by these amazing wings, like sparkling, that we haven’t considered in this post. But I hope the structural color point is now more clear, and that is what I really wanted.

iridescent beetle
Iridescent beetle. Similar but greenish result using a different nanometric structure over its elytra (hard wings).

If you have read until this point, I just can say: wow! And thank you. I have a small present for you, and I swear it’s a beautiful one. Here you have a video that shows this structural color in action in our Morpho butterflies, and also talks about some of the notions I’ve tried to explain here. Please, note the red coloration in the far angled sections of the wings (related to the figure above), and also the brownish areas where you can perceive the real pigmentation of the wings. Enjoy!

References:

[1] K. Yu, T. Fan, S. Lou, D. Zhang. Biomimetic optical materials: Integration of nature’s design for manipulation of light. Progress in Materials Science. 2013; 58: 825-873

[2] S. Niu et al. Excellent Structure-Based Multifunction of Morpho Butterfly Wings: A Review. Journal of Bionic Engineering.2015; 170-189

[3] L. P. Biró. Photonic nanoarchitectures of biologic origin in butterflies and beetles. Materials Science and Engineering. 2010. 169:3-11

[4] Z. Han et al. Structural Colour in Butterfly Apatura Ilia Scales and the Microstructure of Photonic Crystal. Journal of Bionic Engineering Suppl. 2008. 14-19

[5] S. Kinoshita, S. Yoshioka, K. Kawagoe. Mechanisms of structural colour in the Morpho butterfly: cooperation of the regularity and irregularity in an iridescent scale. R.Soc. Lond. B. 2002; 1417-1421

 

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