Radiative Cooling Strategies by Bio-Inspired Nanowire Trees
Spectral thermal radiation properties play a vital role in governing heating and cooling phenomena on surfaces, especially those exposed to the sun in the ambient environment. The wavelength of incident solar radiation responsible for surface heating typically ranges from 200 nm to 2500 nm. Radiative cooling, by means of utilizing the coldness of outer space (~2.7 K), presents a possible solution of avoiding undesirable heat generation. The atmospheric transmission window, which enables radiative cooling in the ambient environment, ranges from 8 μm to 14 μm to reemit incident radiation to outer space. Effective utilization of radiative cooling involves controlling the surface emissivity spectrum, thereby governing the thermal energy exchange. Here, we propose a novel emissivity control technique based on nanowire tree structures that are inspired by the unique colorization of Morpho butterflies. The proposal offers the design of the nanowire tree aided by computational methods which show that the nanowire tree keeps the emissivity below 0.1 in the spectral range between 1 μm and 6 μm, which minimizes solar heating, while keeping the emissivity above 0.8 in the range beyond 8 μm, which maximizes the cooling opportunity in the ambient environment. This enables a near 6°C temperature reduction with respect to the ambient, which is substantially lower than bare surfaces of common engineering materials (e.g. 90% lower than Al2O3). The nanowire trees offer multi-dimensional geometric variations for enhanced tunability of emissivity, providing a significant breakthrough in thermal management methods.