Polyommatus icarus (Lycaenidae) obtains its beautiful pale blue color by diffusion -
© SpaceBirdy - Wikipedia, CC BY-SA
Some pigments in the wings of butterflies are synthesized by the organism itself, while others come from the food plants of the caterpillars, according to a study published by our partner The Conversation.
Butterflies emit light by "fluorescence", meaning that some of their molecules emit visible light under ultraviolet light.
The analysis of this phenomenon was carried out by Serge Berthier, professor of physics at Sorbonne University.
In many civilizations, Nature is more of a source of raw materials than of inspiration, but under the double constraint of our population growth and depletion of resources, things are starting to change.
Harnessing natural resources in a more virtuous, planet-friendly and resilient way - this is precisely how nature works spontaneously.
While we use all of the existing chemical elements, nature uses almost only six: CHNOPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur).
And yet, what efficiency in its achievements!
Nature shows us another way to do it.
Bio-inspiration is one of the very active lines of research today to ensure our future on earth.
A permanent exhibition at the City of Science and Industry presents the foundations of this approach and examples of achievements in all fields.
Color, a universal component of our environment, is a very good example.
Butterflies show us the way!
Color is not a measurable physical quantity, but an impression created by the brain under the effect of different visual signals.
No one, even within the same species, perceives colors in the same way and it is necessary, when one imagines the reactions of butterflies to their own colors, to be wary of any anthropocentrism.
For example, many animal species see in black and white, and those who see in color do not see the same thing as we do.
Insects, for example, see little red, but are sensitive to ultraviolet rays.
Pigments in the scales
Pigments are molecules that absorb part of the visible spectrum to only show the other part.
A pigment that absorbs blue and purple, for example, will only reflect the other colors that make up white light, and will appear rather red.
The pigments provide the vast majority of “warm” colors (from red to yellow).
The larger a molecule, the longer the wavelengths it can absorb.
To make blue (small wavelength), you have to absorb red (long wavelength), therefore large molecules.
This explains why blue pigments are rare in nature and difficult to synthesize.
In butterflies, they are only found in very rare species, such as
Graphium
and some
Nymphalidae
.
Nessaea batesii, a Nymphalidae, is one of the few butterflies with pigmented blue on the fore wings. It is probably a pterobilin, a bile pigment © Serge Berthier, CC BY-NC-ND
The energy absorbed by the pigments is mainly converted into heat.
They therefore also participate in the thermal regulation of butterflies (which heat themselves with solar energy).
The origin of the pigments is multiple: the melanins (black to yellow) are for example synthesized by the organism itself, while others come from the food plants of the caterpillars and participate in the coloring of the "imago" after molting.
Some pigments are toxic and protect the insect against predators - these are the so-called “aposematic” colors.
Emit light
Fireflies and glowworms emit light as a result of a well-known chemical reaction called “bioluminescence”.
Butterflies, on the other hand, emit light by "fluorescence": some of their molecules emit visible light under ultraviolet rays.
The hind wings of the large Philippine butterfly Troides magellanus fluoresce in green-yellow. This effect is added to an impressive visible iridescence at high incidence @ Serge Berthier, CC BY-NC-ND
The fluorescence efficiency is generally low, but it is sufficient in some species of butterflies to modify their appearance under the effect of solar UV, for example in the
Morpho sulkowskyi
, the mythical blue butterfly of the Amazon, or the
Troides magellanus
from the Philippines.
Nanostructures for colorful optical effects
Structural colors, also called "physical colors", are created by various optical phenomena.
These are for example thin film interferences, such as those observed on a puddle of oil on a wet ground for example.
Such stackings of layers are found in many insects, but also on feathers (peacock, pigeon throat) and fish scales (Ablette).
Papilio Neophilus olivencius. Its scales contain three-dimensional photonic structures © Serge Berthier, CC BY-NC-ND
The physical colors can also come from the diffraction by very close line grids, like on a CD, or the diffusion by very small particles, responsible for the blue of the sky, or of our eyes.
The structural colors are not related to absorption (pigment), but to the separation of wavelengths.
They are created by photonic structures, that is to say periodic architectures whose period is of the order of magnitude of the wavelengths of light.
They are generally very bright, directive and saturated;
but if the nanostructures are disordered, the colors are duller and not very saturated.
When men use butterfly tips
Since the dawn of humanity, these are the pigments that have colored our life.
Minor drawback: many are not very stable and degrade under the action of light.
More serious: some, and more and more, are considered as potentially carcinogenic, which prohibits certain applications, in particular in cosmetics.
The idea of making colors without pigments is therefore flourishing!
But these colored nanostructures also have many other properties: antibacterial (bacteria cannot move around there), superhydrophobic (water does not penetrate into the structures: the wing remains dry), self-cleaning.
Nature is thrifty, and here, multifunctional.
Nanostructures observed in nature inspire scientists. Here, a wing of Polyommatus Icarus (Lycaenidae) seen under a microscope. The upper membrane of the scales consists of a spongy structure extremely fine chitin filaments (100 nm) and intermixed © Serge Berthier, CC BY-NC-ND
These are all characteristics that we would like to develop on our artificial surfaces: windows, fabrics, solar collectors.
Apart from the simplest structures such as the stacking of thin layers (in the anti-reflective treatments of our optics for example), we do not know how to make them, and natural structures constitute an infinite source of inspiration.
Without trying to use them directly, we can study them, understand how they work and try to reproduce them.
For example, structures inspired by butterflies are thus found in textile fibers.
How to change the color of an object
While pigmentary colors are immutable, those resulting from a nanostructure depend on its geometry and the otic index of the materials that compose it.
By replacing the air with a fluid around a Morpho, we can make it change color. It is the index of solid / air structures, n, which is modified © Serge Berthier, CC BY-NC-ND
A change in orientation of the nanostructure modifies its geometry (an inclination is enough to modify the thicknesses crossed by the light) and creates iridescence, a color that varies according to the point of view or the incidence of the light.
We find this technique in many luxury paints for boats or cars.
The change of index is easily carried out in solid / air structures.
The color changes are drastic and the technique is used in steam detectors.
A vapor, even in traces, which penetrates a photonic structure induces a measurable and specific change in color of the molecule.
Our "Insects" file
Ideas for our screens?
In the current digital screens of our televisions or smartphones, colored red, green and blue pixels block or allow light rays from a source placed at the back to pass under the effect of an electric current.
This source represents the device's biggest energy consumer.
It is possible to do without it by using, like butterflies, interference pixels which reflect or not the outside light.
Each pixel behaves like a butterfly scale, the thickness of which can be varied using the electric current.
In the absence of electric current, a blue pixel for example has a thickness producing constructive interference in the blue;
with the passage of a current, the thickness decreases and the interferences become destructive for this color.
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This analysis was written by Serge Berthier, professor of physics at Sorbonne University.
The original article was published on The Conversation website.
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