Butterfly-inspired structures on strong and tough metagel for enhanced passive radiative and evaporative cooling

In climates with high humidity and intense thermal radiation from the sun and atmosphere, passive radiative cooling (PRC) alone becomes limited in its cooling potential and will require combination with other mechanisms to enhance cooling performances. By looking into nature, we mimic biological structures and developed a bio-metagel that possesses enhanced radiative and evaporative cooling properties for efficient passive cooling. Inspired by the surface patterning on butterfly wings, the infrared emittance of the bio-metagel is enhanced and can reach 97.1 % within the 8–13 μm atmospheric window. When combined with evaporative cooling, a sub-ambient temperature of 5.7 °C is achieved under direct sunlight in the hot and humid conditions of Singapore with a simulated cooling power of over 320 W/m². Furthermore, the bio-metagel exhibited excellent mechanical properties with ultra-high tensile strength of 6.19 MPa, toughness of 19.09 MJ/m³ and modulus of 2.61 MPa, which could tackle the practical application. Compared to conventional PRC coatings, the bio-metagel can save more than 33 % electricity consumption in building cooling. This work provides insight to the design of high performance passive coolers through bioinspiration for a wide range of applications in diverse climates, thereby leading to energy-savings and the mitigation of urban heat island effects.