Thermoregulate with grace: butterfly wing scale structure

“As we as architects begin to think about designing not an object, but a process inspired by nature to generate objects, in short we will have no constraints. With the processes in our hands that allow us to create structures that we couldn’t even have dreamt of. And at one point we will built them.”
Michael Hansmeyer, TEDGlobal 2012

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Wing anatomy

The wing membranes are translucent, but are partially or fully covered in a dust-like layer of coloured scales. Each scale comprises of a flat plate arising from a single cell on the wing surface.

The scales vary in shape in different species, some being rectangular, while others have a shape similar to tear-drops or plumes. An individual scale typically measures about 50 microns across (0.05 of a millimetre) and 100 microns long, although many are hair-like, and are quite much longer.

There are 3 basic types of scales – pigmentary scales, structural scales, and androconia.

Pigmentary scales are mostly flat and are also known as “ground scales” as they usually form a lower ground layer of color and pattern on a butterfly’s wings.

Androconia are found mainly on male butterflies. At the base of the androconia are tiny sacs containing pheromones. The scent is disseminated via tiny hairs or plumes on the edges of the scales, and used to entice females to copulate.

Structural scales are responsible for the refraction, diffraction and interference patterns of light as it strikes or passes through the semi-transparent structural scales. Almost all butterflies and moths have a mixture of pigmentary and structural scales. The latter are bigger than the pigmentary scales. They overlap them, but the semi-transparency enables the colors of the pigmentary scales to be seen through them. In most cases, the structural scales tend to be architecturally more elaborate – their form seems to be achieved almost as in a digitalized self-formation process.

wing-structure2

Fig. 1 SEM images of micro- and nanostructure of the butterfly wing: a-b) multiple scales covering the wing’s surface; c-d) inner chitin structure of each scale.

Thermal experts

The very delicate, submicroscopic structural scales on butterfly wings are multifunctioning. They are responsible for their aerodynamics, camouflage, warning and attracting other animals, but also they play a major role in the formation of gender and thermal regulation.

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Fig.2 Cross-section of the wing’s scale.

Butterflies are poikilothermic organisms, that is they are unable to regulate their body temperature internally and must therefore rely on the external thermal environment – as they need a temperature of 40°C in their thorax for the muscle motors to function properly. It is well known that they can thermo-regulate by behavioral means, altering their posture to open or close the wings, thus varying the exposure of the dorsal surface to solar radiation, warming the flight muscles or preventing from overheating. Scales play an integral role in the transfer of solar radiation by conduction to the thorax, since without them the temperatures of the thorax are lowered.

Is there, however any evidence to suggest that absorption by scales is anything other than a by-product of solar reflection?

Various species are differently equipped with various reflection and absorptions capabilities, which one can correlate with their distinct lifestyles.

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Fig. 3 Degree of reflection and scale form in butterfly species Pieris brassicae, Gonepteryx rhamni and Pachliopta aristolochiae.

The figure presents spectrum of absorption and reflection for the butterflies of the species Pieris (albino butterfly, white), Gonepteryx (brimstone butterfly, yellow), and Pachliopta (dark), but also the difference between descaled and normal wings. The forms of the scales are shown in the small figures. It is easily visible that the reflecting ability is significantly higher with the white Pieris and lower with the dark Pachliopta. Descaled wings also have a capability to reflect light of only a few percent. This indicates the spectral absorption and penetration of the wings to be the largest with darker color.

Another study compared the cover scales of male sister species, P.daphnis and P.marcidus, which are living in different environments; this time concentrating on their inner structure.

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Fig.4 A, C – P. daphnis, dorsal view and SEM picture of the scale’s structure;  B, D – P.marcidus, dorsal view and SEM picture of the scale’s structure. Comparison shows that the polycrystalline blue cover scale of P. daphnis is structurally much more dense than absorbing structure apparent in P.marcidus species.

It has revealed that scales from P.daphnis (A) inhabiting the low altitudes and relatively warmer climate, was made of a regular crystalline structure, highly reflective in the UV-blue with an iridescent blue appearance. P.marcidus was lacking this structure at relatively higher altitudes and lower temperatures, giving them a dull-brown appearance.

Both structures have been put under identical illumination conditions to record their thermal measurements. Those of darker colored males attained temperature 1,5 times those of lighter appearance. This was due to reduced reflectivity resulted from the absence of crystalline structure, increasing the proportion of solar radiation absorbed in the blue-UV region.

Super black

Radiation effects are reversible. Warmth could then also be released at too high body temperatures using the same structures. Nature usually tends to combine different abilities or properties to enhance the desired effect of the process. The Troides magellanus male butterfly is just a perfect example to prove it. Its wings displays very contrasted colors – yellow fluorescent and iridescent hind wings and ultrablack forewings. The latter manage in the thermal radiation of the butterfly as a negative feedback that is stabilizing the body temperature. Their structure combines of both, structural scales and pigmentary ones to achieve the intensity of its color. The black wings absorb an extreme 98% of the visible range of solar spectrum, which means the large amount of energy is absorbed. However, heat accumulation is dangerous for butterfly that needs to remain a constant temperature of the body in order to be able to fly: therefore it emits infrared radiation as a black body to avoid overheat effect. In comparison to flat wing surface having the same quantity of material, the nanostructure wing shows significantly higher absorption and emissivity.

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Fig.5 Left – Troides magellanus, right – SEM picture of its wing’s structure.

The research shows that these scales are formed in the sort of honey comb structure and take advantage of refraction to trap light, much like a fibre-optic cable.
When light passes between two substances with different ‘refractive indices’ – from water into air, say – some of it becomes trapped in the denser medium. Water has a refractive index of about 1.3 relative to air; the butterfly’s wing tissue has an reflective index of about 1.6 relative to air. This maximizes the chance that light will be absorbed by the pigment.

The researchers tested this by immersing the wings in a chemical bath of bromoform, which has roughly the same reflective index as the wing tissue. “We effectively removed the effect of the scales’ structure,” they said.

As expected, the scales looked less black when their structure was canceled out. In air they absorbed more than 90% of light; in bromoform the figure dropped to just over 50%.

This means that the variation is found as well in layer thickness, and demonstrates that these species has evolved multilayer reflectors/absorbents accordingly to their thermal environments, enabling them to elevate their body temperature.

What I personally find extremely fascinating is that the same structures are used for both heating and cooling, which is exactly what seems to be necessary to survive in the extreme arid climate of the desert – to be able to avoid intensive solar radiation during the daytime and absorb heat for much colder nights.

How can we become as smart as butterflies?

Therefore, I started to think of a multilayer structural façade system which would be based on the same principle. What if we could invent  a process of cooling and heating by means of reflection and absorption of solar radiation, depending on the time of the day and season of the year? Would it be possible to perform it using the same structure and the same set of materials?

The figure below shows a proposal that would use a natural air convection together with thermal expansion and retraction. It is a kinetic-like system, that would not be using any motors to support it.

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Fig.6 Left – daytime/summer; right – evening/winter.

During the summer or the daytime when sun is in the high position an we experience intensive solar radiation, as shown in the pictures on the left, the air from inside of the building would be redirected to the outside by natural convection in order to remain inner comfort for habitants. Combined together with the reflection caused by the geometry of the expanded material from the outer layer of the facade, unpleasant heat would remain outside of the building.

As the desert climate is characterized by the high range of daily temperature altitudes, it would be more than convenient to be able to absorb some warmth before it gets dark and cold. Therefore, using the same air flow tunnel and by some openings in the outer layer of the structure after its retraction, heat could be transported towards the inside of the building.

Possible material solutions

Doris Kim Sung, USC architecture professor, during her TEDtalk in 2012 introduced her research on thermo-bimetal. The showpiece of her developmental work takes inspiration from spiracles, a method that grasshoppers use to regulate their body temperature. A honeycomb-like structure can be built that when treated with the bimetal might allow air to flow through a building’s walls instead of opening windows.

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Fig.7 “Metal that breaths” – Doris Kim Sung

Thermo-bimetal requires no control and no energy to manipulate itself. The material is a lamination of two different metals and two different coefficients of thermal expansion. When heated one of the metals will expand faster than the other and result in a curling displacement.

Development of the thermo-bimetals included many different shapes and configurations, and a finger-like set of strips is the current method being tested. The idea is that the material will act as a sun shading device and a ventilation system. When the sun hits the surface of the structure each individual finger moves toward the heat. Each finger is calibrated to get the desired performance based on location within the piece itself and orientation toward the sun.

Metal that breaths – video

_Joanna Sabak

REFERENCES:

Bibliography:

Herman A., Nanoarchitecture in the black wings of Troides magellanus: a natural case of absorption enhancement in photonic matererials, Belgium

Gorb S.N., Functional Surfaces in Biology: Little Structures with Big Effects, Springer 2009

Nachtingall W. and Pohl G., Biomimetics for Architecture and Desing, Springer 2015

Nanooptics in biology

Wing anatomy

The physics of butterfly wings

Passive solar heating and cooling

Super black – carbon film

Doris Kim Sung TEDtalk

AskNature

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