<b>Bio-inspired Strategies for Efficient Radiative Cooling</b>
Creators: name: Felicelli, Andrea Lorena
Publisher: name: Purdue University Graduate School
Year: 2025
Subjects: classificationCode: 401801
subject: Micro- and nanosystems
classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)
classificationCode: 401810
subject: Nanoscale characterisation
classificationCode: 401102
subject: Environmentally sustainable engineering

description: In recent years, the world has witnessed a growing trend of record high temperatures, heat waves, and extreme weather events due to climate change. Thus, there is an urgent need to develop technologies that enhance quality of life while mitigating further contributions to climate change. Radiative cooling, a passive cooling technique, offers a promising solution to this challenge. Nature serves as a vast, largely unexplored source of inspiration, with various biological systems utilizing radiative cooling to thrive in extreme environments. This work looks at what can be learned from nature to better develop radiative cooling technologies.While nanoparticle-based coatings and biologically-inspired nanocellulose-based structures have shown promise in radiative cooling, each has its limitations. Nanocellulose-based structures exhibit high mechanical strength but lower solar reflectance due to UV absorption. On the other hand, nanoparticle-based coatings require a high volume of nanoparticles, resulting in brittleness. This work introduces a dual-layer system comprising a cellulose-based substrate and a thin nanoparticle-based radiative cooling paint, maximizing both radiative cooling potential and mechanical strength. The relationship is studied between thickness and reflectance of the top coating layer with a consistent thickness of the bottom layer. The saturation point is identified and used to determine the optimal thickness for the top-layer. With the use of cotton paper painted with a 125 microns BaSO<sup>4</sup>-based layer, the cooling performance is enhanced to 149.6 W/m<sup>2</sup> achieved by the improved total solar reflectance from 80% to 93%.Looking at another source of biological inspiration, radiative cooling potential of the white shell of the <i>Sphincterochila</i><i> zonata</i> desert snail is investigated through experimental techniques, revealing a remarkable 90.8% total solar reflectance and 0.88 sky window emissivity, which is achieved through nanoscale features and layered platelet-like morphologies. This is a record high for a biological system. The porosity, nanostructure, and material composition are analyzed, and compared to relative biological systems in other white shells, including those living in the same Negev desert and highly contrasting ocean dwellers. Structural analysis demonstrates layered platelet-like morphologies that optimize for light scattering in solar wavelengths. We investigate the shell's porosity, nanostructure, and material composition through comparison with other species’ shells in the Negev desert and marine environments. Through this, we gain inspiration from <i>Sphincterochila zonata</i> to develop our own radiative cooling technologies.In weight-sensitive applications, thin and lightweight radiative cooling paints are crucial, but achieving high solar reflectance remains a challenge. Using inspiration of the layered structure seen in desert snails, this research introduces ultrawhite hBN-Acrylic paints that achieve a remarkable solar reflectance of 97.9% with only 150 µm thickness and 0.029 g/cm<sup>2</sup> weight. The unique properties of hexagonal boron nitride (hBN), including a high refractive index and nanoplatelet morphology, enable a combination of Mie and Rayleigh scattering, while a 44.3% porosity enhances refractive index contrast. Field tests demonstrate that hBN-Acrylic paints provide full daytime cooling under direct sunlight, reducing temperatures by 5-6℃ below ambient.Furthermore, biodegradable chitosan-hBN films are introduced as a promising advancement in sustainable cooling technology. These films, composed of up to 60% hBN nanoplatelets within a chitosan matrix, offer flexibility, mechanical robustness, and significant cooling potential. Preliminary results show that these films achieve high solar reflectance and maintain structural integrity, with further potential for optimization through nanoplatelet alignment techniques like hot pressing. By integrating bio-inspired and synthetic approaches, this work contributes to the broader goal of developing sustainable, high-performance materials for passive cooling.