Morpho Butterfly-Inspired Spectral Emissivity of Metallic Microstructures for Radiative Cooling

Radiative cooling offers unique capabilities of controlling surface temperature and improving the energy efficiency of various systems ranging from buildings to micro-devices and personal clothing using re-emission of heat via the atmospheric transmission spectrum in the spectral range of 814 μm. Control of the spectral emissivity profile results in control of the thermal behavior of the surfaces. Tailoring the emissivity profile to match the atmospheric transmission spectrum results in radiative cooling of surfaces. Here we investigate the use of bio-inspired designs for radiative cooling applications. Our simulations of Morpho butterfly inspired branched microstructures (micro-trees, briefly) based on rigorous coupled wave analysis offer optimally-designed micro-trees that act as perfect reflectors of incident solar radiation, while acting as near-black bodies within the atmospheric transmission spectrum. The structural periodicities result in an emissivity below 0.2 between 1 μm and 6 μm, reducing adverse effects of solar heating. Meanwhile the emissivity remains above 0.7 for 8-14 μm, offering an increased opportunity for radiative cooling. The use of typically hot metallic surfaces, with very low emissivity values, results in radiative cooled metallic micro-trees with spectral-selective emissivity. The surface temperature of metallic micro-trees is roughly 9-10 K lower than bare metallic surfaces, aided by an increased cooling power of roughly 140 W/m2. The spectral selectivity of these structures is attained by the tunable morphology of the structures themselves. The bio-inspired micro-trees present opportunities for passive radiative cooling, with their structural flexibility and spectral tuning opening the field for broad thermal management applications.