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Cool like a butterfly wing

New study demonstrates that butterfly wings contain a network of living cells with specific thermodynamic and thermoregulatory properties. Butterfly wings cool down as a result of low solar absorption combined with efficient radiative heat dissipation.

Butterflies wings are very sensitive to heat. Excessive sunlight exposure could cause their wings to overheat while too low temperatures could cause them to freeze. A team of engineers and biologists from Columbia Engineering, Harvard university and Washington university has identified a network of mechanical and temperature sensors inside butterfly wings.


A butterfly wing absorbs sunlight and exchanges energy via thermal radiation with the ambient environment, including the terrestrial environment beneath the wing and the sky above. The radiative energy emitted from the heated wing to the cold sky represents a major channel of heat dissipation in addition to heat losses via convection.


The research team examined butterfly wings under wavelengths of light ranging from the ultraviolet to the visible to the mid-infrared. In the visible, the coloured parts of wings are associated with absorption and reflection of sunlight. Meanwhile, in the near-infrared, butterfly wings have lower solar absorptivity. Since sunlight is about 50% less powerful in the near-infrared, lower near-infrared absorption helps to reduce the overall temperature of the wings.


Furthermore, different parts of the butterfly wing have different thermal emissivity. The parts containing living cells have high thermal emissivities approaching unity; which is perfect for heat dissipation through thermal radiation. Hyperspectral imaging has shown that the average temperature of the parts containing living cells is always lower than that of the “lifeless” parts of the wings. For the “living part” of the wing, high thermal emissivity is caused by scales with unique nanostructures along with thick membrane layers.


In addition to these structural aspects, butterflies can also react to the intensity and direction of sunlight. They move in ways that allow them to displace thermal stimuli applied to their wings. Simulated environmental conditions using laser beams have shown that these reactions occurred whenever a laser spot was directed at the “living” components in the wings; suggesting that they may contain a network of thermal sensors.


This research could lead to various applications for temperature distribution in objects that are lightweight and translucent like butterfly wings. Nanostructures found in the wing scales could inspire the design of radiative-cooling materials to help manage excessiveheat conditions.


Source: Tsai, C., Childers, R.A., Nan Shi, N. et al. Physical and behavioral adaptations to prevent overheating of the living wings of butterflies. Nat Commun 11, 551 (2020). https://doi.org/10.1038/s41467-020-14408-8

 


Fig 1: Thermodynamics of butterfly wings
© Nature

“Steady-state wing temperature is the result of a balance between absorption of sunlight (orange arrow), convective heat loss to the surrounding air (purple arrows), and radiative energy exchange between the heated wing and the relatively cold surrounding environment (red arrows).”


Fig 2: Temperature distributions on the forewing of three species of Eumaeini butterflies illuminated by sunlight. 
© Nanfang Yu and Cheng-Chia Tsai
Despite wide variation in visible wing coloration and pattern, the temperature of the scent patches, pads and wing veins that contain living cells is always lower than that of the remaining “non-living” parts of the wings.