A demonstration experiment on structural color was introduced a long time ago: spraying some alcohol /isopropanol on the surface of blue butterfly wings, the butterfly wings will turn from bright blue to darker green after they are soaked from these alcohols. and when the liquid evaporates, the original color will be restored.

to understand this phenomenon, we must first talk about how the blue on butterfly wings is produced. This is a structural color, that is to say, it is not derived from the pigment, but from the optical phenomenon produced by the specific microstructure. The "optical elements" on the butterfly's wings look roughly like this. There are many small parallel structures where light reflects and interferes between these interfaces. As a result, the reflected light of a particular color is enhanced and finally appears blue.

and ethanol can infiltrate into the gaps of these structures, replace the air in them, change the refractive index of the gap, and the structural color of the "optical element" changes accordingly.

this is generally the end of the demonstration, but recently it has been found that there is a more in-depth discussion of this experiment, so let's share some supplementary information.

if you drip water on a butterfly's wings instead of ethanol, what color will the blue butterfly's wings become? The answer is-the color doesn't change at all.

these optical microstructures also create a good hydrophobic effect on butterfly wings, so when water is dripped directly, water can not infiltrate into the microstructure at all, and air still exists between the gaps in the microstructure. (of course, the hydrophobic surface of butterfly wings is very reasonable.)

(the effect of adding water directly)

however, this paper also provides a way to allow water to infiltrate: pretreatment with surfactants to increase the hydrophilicity of wings. Specifically, a Nonionic surfactant Triton X was used. First treat butterfly wings with methanol solution dissolved with surfactant, wait for the solvent to evaporate and then add water, and the water can infiltrate into the microstructure. This time the discoloration effect is as follows:

using this method, sugar water with higher refractive index can also be infiltrated into the microstructure. The following is the effect of infiltrating saturated sugar water:

in addition to the blue butterfly, the authors also tried a similar treatment on the wings of the sunset moth. Adding ethanol to the wings of the sunset moth can turn the yellowish green structural color into red, and the effect is also very beautiful:

original text:

https://pubs.acs.org/doi/10.1021/acs.jchemed.7b00463

if you are interested, you can take a look at

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PS: this journal is Journal of Chemical Education, the theme is how to use more interesting methods and experiments to promote education, the above content is very interesting ~