Light-Absorbing Wings: The Secret to Super Black Butterflies

"The white shores and beyond them the distant green hills under the rising sun" is one of the most famous descriptions of the "continuation of the path" expressed by Gandalf in the film "The Lord of the Rings." If we discard the philosophical subtext, this picture will seem banal to some, and incredibly beautiful to some. Here, as they say, to each his own, for beauty is in the eye of the beholder. No matter what a person likes to admire, whether it be a sunset or a sea surface, the physics of these objects of aesthetic pleasure remains immutable. Different materials interact with light in different ways, which gives rise to such a wide variation in the color spectrum. But there are materials that practically do not reflect light, absorbing most of it. Of the synthetic materials, it is worth highlighting Vantablack, which absorbs up to 99.965% of radiation. So scientists at Duke University found outthat the wings of some types of butterflies have quite comparable characteristics, while being several times thinner than any of the substances created by man. How black are butterflies' wings, why do they absorb radiation, and how can these findings be put into practice? A report by scientists will tell us about this. Go.

Study basis

When it comes to creating a “super-black” material that absorbs almost all radiation, the main focus is not on chemicals, but rather on their internal architecture. So, for example, Vantablack is made of vertically aligned carbon nanotubes. However, such a material is extremely fragile, which greatly limits the range of its application.


A small video about Vantablack.

In contrast to synthetic materials with a high absorption coefficient and, accordingly, a low reflection coefficient, there are natural ones (coal, soot, platinum black, etc.).

If we talk about living things, and not about different minerals, then here nature also does not graze the rear. In some species of horse spiders, birds and butterflies, the color is comparable in properties with Vantablack.

So the reflectivity of the wings of butterflies from the Papilionidae family is 0.2%, which is due to their polydisperse honeycomb structure. And here the question arises - do all species from the Papilionidae family have the same wing structure?


Pachliopta kotzebuea is a species of butterflies from the Papilionidae family.

Butterfly wings, despite their fragility, are quite complex in their structure, and this is a tendency inherent in all species from all families. The bottom line is that this structure can differ and provide for the implementation of various physical processes.

A detailed examination of the butterfly wing Papilionidae allows us to see that the absorption of radiation is due to two layers of microscopic scales, the size of which sometimes does not exceed 2.5 microns. A plate with a pattern in the form of a quasi-honeycomb structure consisting of transverse ribs connecting the edges is located on the upper part of the scales.


Butterfly wing flakes as exemplified by the species Gonepteryx cleopatra (Clematra lemongrass).

Such a lively interest in butterflies in the aspect of studying ultra-black materials is due, according to scientists, for a number of reasons: butterflies have several types of scales different in their specialization, including multilayer reflectors and thin films; scales are many times thinner than other ultra-black natural materials and synthetic analogues; The flakes are light and durable enough to allow flight.

Previous studies have shown that nanoscale (240 nm) holes in the upper plate of flakes allow light to penetrate inside the flakes, where it is absorbed by melanin, which binds to chitin in the cuticle. It was also found that such plates absorb more light than plates without holes. But it has not yet been possible to precisely determine the purpose of the structural ribs restricting these holes. Some scientists believe that such a structure provides an increase in light absorption due to the direction of light in the hole.

In the study we are considering today, scientists decided to examine these mysterious scales in more detail in order to more accurately describe both their structure and their light-absorbing properties. For this, spectrophotometry, scanning electron microscopy (SEM), and finite time difference modeling in the time domain were used.

Preliminary findings are that despite significant differences in the size and shape of the nanoholes, all ultra-black scales have steep edges and trabeculae, which are much deeper and wider than ordinary black or brown scales. Optical modeling has confirmed that these structural features are key in such a low reflection coefficient, and that the removal of any of them sharply increases the reflection coefficient by more than an order of magnitude. It is also apparent that such a flake structure increases the area for light absorption. In an attempt to explain the practical application of ultra-black wings, scientists put forward a theory: since all butterflies have super-black sections of the wing that always border on areas of a different color (white, red, etc.), this may be necessary to increase the perceived brightness and color saturation,what is applied in interspecific and intraspecific communication.

Research results

To study the potential diversity of the nanostructures underlying the super-black color of butterflies, scientists selected 10 species of butterflies from four subfamilies: Papilioninae , Biblidinae , Danainae and Heliconinae . Butterflies of brown and ordinary black colors were also selected as a control group.


Image No. 1: reflectivity of different groups of butterflies depending on the radiation wavelength.

In all butterflies from the super-black group, the reflectivity was from 0.06% to 0.4% with perpendicularly incident light (500 nm). In butterflies from the control group, this parameter ranged from 1 to 3%.


Image No. 2: morphological diversity of the structure of scales of the wing of butterflies from the super-black group.

Ultra-black materials ultimately depend on absorbing pigment embedded in a complex structure. First, an assessment was made of the structure of 11 butterflies (7 superblack, 4 control) using SEM. In all butterflies, the flakes had an upper layer perforated with quasiperiodic holes. There were significant differences in the shape and size of the holes: in the form of a chevron in Eunica chlorocroa ; rectangular openings (500x330 nm) for Catonephele antinoe , Catonephele numilia and Heliconius doris ; rectangular openings (750x500 nm) for Euploea Dufresne and Euploea Klugi .

It is noteworthy that not one of the nymphalid butterflies (subfamilies Biblidinae , Danainae and Heliconinae ) had a honeycomb structure similar to that found in papillionids (family Papilionidae ).

The absence of a honeycomb structure in combination with a variety of size and shape of the holes suggests that the light absorbing ability of butterflies does not depend on the specific shape or size of the holes. However, some of the individual changes in the shape of the hole probably help to increase absorption at abnormal angles of incidence of light.


A variety of shapes and sizes of nanoholes by species among ultra-black butterflies: A - Catonephele antinoe; B — Catonephele numilia (); C — Catonephele numilia (); D — Eunica chlorocroa; E — Euploea dufresne; F — Euploea midamus; G — Euploea klugi; H — Heliconius doris; I — Heliconius ismenius; J — Napeocles jucunda; K — Trogonoptera brookiana (); L — Trogonoptera brookiana ().

Two common structural features were found in all samples of superblack butterflies: steep edges and durable trabeculae connecting the upper and lower plates. In the samples from the control group, the openings were much larger, and the trabeculae were either much smaller or completely absent.


Trabeculae (highlighted in red) in males from superblack species of butterflies are larger than in females from the control group: A - Trogonoptera brookiana (female); B - Trogonoptera brookiana (male); C - Catonephele numilia (female); D - Catonephele numilia (male).

These differences directly indicate the importance of the revealed structural features to obtain a low reflectivity of the wings.



It is curious that the wings of ultra-black butterflies, where nanoholes and large trabeculae were found, even after covering them with gold (for conducting REM studies) still retained their black color (photo above).

The observations made it possible to obtain valuable data that were used in the simulation: the contribution of the identified geometric features of the butterfly wing to the formation of reflectivity was evaluated.

Since the wing flake can have low reflectivity and even be transparent (like moths), the reflectivity was calculated on two overlapping flakes lying on a 100% white surface.

Using the finite time difference method, the reflectivity of a structure with rectangular openings of 500x330 nm was modeled for a butterfly of the species C. antinoe (the blackest among all the studied species).

Then a similar simulation was carried out, but with removed ribs, trabeculae or basal plates. As a result, a butterfly wing flake model was obtained with a reflectance in the range from 0.4 to 1.0% over the entire visible part of the spectrum.


Image No. 3: simulation results. Designations of colors: blue - a model of a full-fledged scale; red - flake with removed trabeculae; yellow - flake with removed ribs; Violet- with removed basal plates; brown - with removed trabeculae and ribs; green - a rectangular block of the same volume of absorbing material as in a conventional flake.

This indicator was 14–40 times lower than in the case of modeling using two flat overlapping plates of the same absorbing material as the flakes.

That is, the removal of the ribs or the entire internal structure of the flake led to an increase in reflectance by 3–16 times. If you remove a comparable volume of reflective material without touching the geometry of the flake itself, then the reflectivity increases only twice. As for the removal of basal plates, the changes in this indicator were extremely insignificant.

Next, it was decided to consider the contribution of geometry to the formation of reflectivity without the participation of absorbing material. For this, scales with transparent ribs and trabeculae were modeled.

This test showed that only the edges of the flake structure reduce the reflectivity by 14-58% compared to flakes in which there are no edges at all. Trabeculae, however, reduce reflectivity by 5-70%, compared with flakes where they are not at all.


Image No. 4: comparison of the reflectivity of flakes without trabeculae and with transparent trabeculae (left); comparison of the reflectivity of flakes without edges and with transparent edges (right).

After analyzing the geometry, scientists began to analyze the melanin pigment in order to find out the importance of its optical properties in the formation of the general reflectivity of the butterfly wing. Melanin has an unusually high real ( n ) and imaginary ( k ) refractive index for both biological material.

Using morphological features of the structure of flakes, reflectivity was simulated at 550 nm using 99 unique combinations of the real and imaginary parts of the refractive index. The measured real part of the refractive index was from n = 1.33 (water) to n = 1.8 (melanin), and the imaginary part from k = 0.0 (without absorption) to k = 0.20.


Image No. 5: reflectivity is mainly determined by the imaginary part of the refractive index.

Without absorption, the reflectivity of two flakes located on a white background approaches 100%, but at k = 0.06 it begins to decrease by 1%. The effect of the real refractive index dominates the action of the imaginary up to k > 0.06. For a flake with a high imaginary part ( k > 0.10), the situation changes and the real part of the refractive index becomes responsible for the increase in reflectivity. For example, when k = 0.15, the reflectivity at n = 1.33 is 88% lower than at n = 1.8. When 0.06 < k<0.10, reflectance depends on both components of the refractive index.

This suggests that to achieve the reflectivity observed in ultra-black butterflies, melanin is not needed, only absorbing material is needed (ideally, the material with the real part of the refractive index is lower than that of melanin).

For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.

Epilogue

This study showed that butterflies of some species have unique nanoscale structures in their wings, which provide a reflectivity of not more than 0.06% of radiation at an angle of 90 °.

It was found that between the flakes covering the wings of butterflies, there are trabeculae that increase the surface area to absorb radiation from cuticular melanin.

Thus, this super-black structure is comparable to synthetic ones, except that it is only 1/5 of their thickness. Therefore, if we take the wings of butterflies as a basis, we can try to recreate a certain synthetic analogue that will have similar reflectivity and be equally thin, but strong.

According to the researchers themselves, their work is of great interest not only to engineers and nanotechnologists, but also to zoologists (in particular, lepidopterologists studying butterflies), since it remains unclear why butterflies are so complex. Perhaps such an unusual color is necessary to attract the attention of a partner or to scare away predators, there is no exact answer yet. Scientists also intend to continue their research, focusing on the evolutionary component. They want to find out how much super-black color in the wings of butterflies manifested itself along their evolutionary path. In this way, they will be able to understand why it is the currently observed structure of the scales that became dominant, and also what influenced its formation.

No matter what scientists find out in the future, their work has become yet another confirmation that nature still has something to surprise us with.

Friday off-top:

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Thank you for your attention, stay curious and have a great weekend everyone, guys! :)

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