How nature uses physics to create blue

Pigments are one way to be colorful, but butterflies rely on physics at the nanoscale.

radwanThe Blue Diadem butterfly, found on the African continent, is roughly the size of a saucer with wings spread. These wings fascinated Radwanul Hasan Siddique of the California Institute of Technology. It wasn’t pigment that turned the butterfly’s wings a radiant cornflower blue, but what was it? He found out, and is applying lessons he’s learning from nature’s palette to biomedical devices.

Follow his current project on the development of one such device. 

ResearchGate: Why is the color blue so rare in nature, and is this actually true?

Radwanul Hasan Siddique: I don’t think blue is rare in nature. The sky is blue, so are several butterflies, birds, spiders, even some fruits. Still, this question arises quite often, and the main reason is the rarity of blue pigments in nature. Biological pigments are mainly organic compounds that selectively absorb incoming light and reflect the remaining wavelengths appearing as color. The color blue is shorter in wavelengths that are higher in energy (frequency) compared to green and red. Blue light provides sufficient energy to raise an orbital electron to an excited state and hence forces organic molecules to absorb it. So most of biological pigments absorb blue and therefore appear more green and red in color. If you want to be blue, the simple answer could be you don’t want to absorb blue.

Blue diadem butterfly, Hypolimnas salmacis. Credit:Bernard Dupont
Blue diadem butterfly, Hypolimnas salmacis. Credit:Bernard Dupont

RG: What are the pros of being blue?

Siddique: Nature evolved its colors depending on the environment. Usually blue is brighter than any other color in nature. This can be a pro if you want to stand out from others. For example, blue butterflies use their blue to attract mates.  A nice example is bright blue Pollia fruit, that comes from Africa and attracts pollinators to carry its seeds around the world.

RG: You studied blue tarantulas and blue butterflies. Why?

Siddique: Due to the rarity of blue pigments, nature uses often pure physical phenomena to create blue. This is called structural color. Structural color comes from the light interacting with micro- and nanostructures on the cell wall or the exoskeleton of the organism it hits. Fundamental optical processes such as reflection, refraction, interference, diffraction and scattering can become sources of structural colors. Most structural colors are enhanced by combining these optical phenomena. As an engineer and applied physicist, it is particularly intriguing for me to study how nature engineers photonic/optical structures to create blue.

RG: What makes the Blue Diadem butterfly blue?

Siddique: The Blue Diadem butterfly has white, blue, and brown-black patches on its wing. Roof-shingle like scales on its wings create this coloration. A scale usually has two parts: an upper lamina and a thin bottom laminar film. The Blue Diadem’s bottom lamina is perfectly designed to create blue. It’s just the right thickness. The upper, more structured and complex lamina just acts like a translucent window, allowing the blue color to pass through. When light hits a scale, the bottom 200 nm thin laminar film causes a phase delay of light waves. This phase delay causes superposition of waves, also called constructive interference, which appears as blue (wavelength of 400 nm) and destructive interferences in other visible spectra.

Interference of two waves. Credit:Wikipedia
Interference of two waves. Credit:Wikipedia

RG: How did you study this?

Siddique: The scales are almost 1000 times smaller than a hair. We used a scanning electron microscope and transmission electron microscope to examine the fine details of the scales in nanometer resolution. Afterwards, we used special spectroscopy techniques to analyze the reflected, transmitted, and absorbed light by different scales. Finally, we came up with a theoretical model using the experimental results and explained the Blue Diadem butterfly’s proper physics.

RG: Is it different from other blue butterflies?

Siddique: Yes, it is. Most blue butterflies use the more complex structures in upper lamina to create a blue coloration. Blue Diadem butterflies use the lower lamina. For example, the famous Morpho butterflies get their blue color from light interacting with the ‘Christmas tree’ like structure of its upper lamina. The bottom lamina doesn’t play any significant role. Usually, if you look at a single Morpho butterfly scale, you will see the blue color quite nicely. In the case of Blue Diadem, you won’t see blue, it looks white. That is what I found fascinating and what started my research.

Morpho butterfly. Credit: Kevin Walsh
Morpho butterfly. Credit: Kevin Walsh

RG: What happens with your findings? What is their application?

Siddique: I call what I do bio-inspired nanophotonics. I try to understand the underlying physics of fascinating optical structures found in nature. The next step is to develop affordable nanofabrication techniques to apply these findings, and incorporate them into device-level biomimetic engineering solutions for multi-functional opto-electronic and bio-medical applications.

For example, my current project is to develop a flexible and convenient monitoring solution for intraocular pressure (IOP) to improve glaucoma management. Glaucoma is a leading cause of irreversible blindness in 60 million people worldwide.  An elevated intraocular pressure (IOP) level has been identified as a major risk factor, and all the glaucoma therapies are aimed at lowering the IOP level. My project focuses on developing a nanophotonic sensor that’s able to detect IOP, perform long-term and is biocompatible. As my sensor needs to be efficient at various dimensions, I am letting myself be inspired by biological photonic structures that nature uses to optimize herself for different functionalities in the best possible manner.

Feature image: Morpho didius. Didier Descouens