Read on Twitter
A thread on structural color! 
In the video isopropyl alcohol is dropped on a wing from a Blue Morpho butterfly. But why does it change colors? Why is it even blue in the first place?
The answer lies in the nano-structures found on the wing.
In the video isopropyl alcohol is dropped on a wing from a Blue Morpho butterfly. But why does it change colors? Why is it even blue in the first place?
The answer lies in the nano-structures found on the wing.
If you look at a butterfly wing under an electron microscope at high magnifications you can see thin periodic lamella structures lining the surface of the scales, that have a spacing of about ~100 nm.
A periodic structure alternating between different refractive indices at this size scale has unique properties because it is a photonic crystal. This means that it has a band gap where certain wavelengths of light cannot propagate through. Below is a 1D photonic crystal structure
The structure on the Blue Morpho's wings is a specific type of photonic crystal: a distributed Bragg reflector! If the periodicity of the structure is about ~1/4 of the light's wavelength, the waves are not only forbidden from propagating but they actually reflect very well!
The reflectivity of a certain wavelength off of the wing is dependent on the refractive indices of the ridged structures of the wing (made of chitin) and the surrounding medium (usually just air, n=~1).
We can't change the refractive index of the ridged structures, however we can drop liquids with a different refractive index than air (in this case IPA, n=~1.4).
This is what causes the awesome color changing effect shown in the video!!!
This is what causes the awesome color changing effect shown in the video!!!
