Enhancement of infrared emissivity by the hierarchical microstructures from the wing scales of butterfly Rapala dioetas

Radiative cooling, which normally requires relatively high infrared (IR) emissivity, is one of the insects’ effective thermoregulatory strategies to maintain their appropriate body temperature. Recently, the physical correlation between the delicate biological microstructures and IR emissivity for thermal radiation draws increased attention. Here, a scent patch region on the hindwing of Rapala dioetas butterfly is found to exhibit enhanced IR emissivity compared with the non-scent patch regions. A series of optical simulations are conducted to differentiate the effect of biological structures and material composition on the high IR emissivity. Besides the intrinsic IR absorption (emission) of chitin (the main composition of butterfly wings), the hierarchical microstructures of the scent patch scale further improve the IR absorption (emission) through the increased inner surface area and multi-scattering effect. This enhancement of IR emissivity enables the butterfly to efficiently radiate heat from the scent patch region to the environment with a limited volume of chitin. This study of the correlation between IR emissivity and microstructural designs may offer additional pathways to engineer bioinspired materials and systems for radiative cooling applications.