Bioinspired\nSwitchable Passive Daytime Radiative Cooling\nCoatings

Abstract

Passive\ndaytime radiative cooling (PDRC) relies on simultaneous\nreflection of sunlight and radiation toward cold outer space. Current\ndesigns of PDRC coatings have demonstrated potential as eco-friendly\nalternatives to traditional electrical air conditioning (AC). While\nmany features of PDRC have been individually optimized in different\nstudies, for practical impact, it is essential for a system to demonstrate\nexcellence in all essential aspects, like the materials that nature\nhas created. We propose a bioinspired PDRC structure templated by\nbicontinuous interfacially jammed emulsion gels (bijels) that possesses\nexcellent cooling, thinness, tunability, scalability, and mechanical\nrobustness. The unique bicontinuous disordered structure captures\nkey features of Cyphochilus beetle scales, enabling a thin (130 μm)\nbijel PDRC coating to achieve high solar reflectance (≳0.97)\nand high longwave-infrared (LWIR) emissivity (≳0.93), resulting\nin a subambient temperature drop of ∼5.6 °C under direct\nsunlight. We further demonstrate switchable cooling inspired by the\nexoskeleton of the Hercules beetle and mechanical robustness in analogy\nto spongy bone structures.