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Photonics

Chemistry in Pictures: Butterfly effect

by Manny I. Fox Morone
July 28, 2020

Credit: Tommy Technetium via YouTube
20200728lnp20-micro.jpg
Credit: GE Research

These wings from a Morpho butterfly are normally bright blue thanks to structural color. Because the nanosized spacing between the wings’ scales (micrograph) matches blue light’s wavelength (between 400 and 500 nm), when white light hits the wings, this iridescent blue is amplified while other colors are tamped down. However, when YouTuber Tommy Technetium gets the wings wet and the liquid fills those small gaps between the scales, the liquid warps the light’s wavelengths such that 400–500 nm wavelengths correspond to a greener color, which is what we see when liquid nitrogen is poured onto the wings (above).

Other liquids create different colors, like the isopropyl alcohol solution in the video below and the other liquids in the multipanel image. Those other liquids are as follows: (a) air; (b) n-hexane; (c) ethanol; (d) water; (e) 400:1 methanol/Triton X solution before methanol evaporation; (f) Triton X-treated wing after methanol evaporation; (g) Triton X-treated wing with water; (h) Triton X-treated wing with saturated sugar water; (i) Triton X-treated wing gently rinsed and wet with water; (j) acetone; (k) toluene; (l) water.

Credit: Tommy Technetium via YouTube

Credit: Tommy Technetium (videos, Subscribe to his YouTube channel, and follow @pchemstud on Twitter); GE Research (micrograph); J. Chem. Educ. 2018, 10.1021/acs.jchemed.7b00463 (multipanel image)

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20200728lnp20-multipanel.jpg
Credit: J. Chem. Educ. 2018, DOI: 10.1021/acs.jchemed.7b00463

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Comments
Mas Subramanian (July 29, 2020 4:19 PM)
The change in the color from blue to green is due to the index of refraction of the medium filling the the Morpho butterfly nanostructure layers. When air is replaced it with alcohol, which has a higher index of refraction, so the reflected color shifts into the green. The wavelength corresponding to green is 500 to 565 nanometers not 400-500 nanometers as mentioned in the article.
Manny Morone (July 30, 2020 4:15 PM)
Hi Mas, Every chart I've seen shows blue light in the 400–500 nm range, and this is the color that the butterfly's wings amplify when they are not wet.

What you've about the indices of refraction of the liquids compared with the indices of refraction of air is correct. Expanding on that, when the light enters the liquid medium, the wavelength of the light is reduced (I use the term "warped") proportional to the index of refraction (λ = λ₀/n). But the physical dimensions of the wing's nanostructures are not affected, so the still amplify 400–500 nm light regardless of the medium. Under the liquid, this range corresponds to green/yellow light (450 = 535/~1.2), which is what is reflected back by the wing, and why we see the color we do.

Another interesting question is, Why does the wing turn brown rather than getting more red? I'm not sure, but I believe this has to with higher-index liquids approaching the index of refraction of the nanostructures (about 1.5), which nullifies the structural color effect, revealing the wing's true color: brown.

Here's a great reference, and it runs through a demo, though a part of me feels bad for the butterflies: https://pubs.acs.org/doi/10.1021/acs.jchemed.7b00463.

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