Electrospinning to Surpass White Natural Silk in Sunlight Rejection for Radiative Cooling

Abstract

Natural white silk cocoons exhibit strong broadband optical scattering by Anderson localization within the silk fibers, providing a low‐light environment to the pupae inside. This scattering effect is due to thousands of densely packed parallel fibrillar nanovoids running within each fiber along the fiber axis. Herein, to enhance sunlight rejection from white silk, conventional diffusive optical transport without Anderson localization is used. For optical diffusion, natural white silk is restructured by electrospinning to destroy the fibrillar nanovoids. Sunlight rejection power of the electrospun structures is controlled by the fiber diameters. Relative to a nonwoven raw silk fabric, a restructured silk film with a mean fiber diameter of a quarter micron substantially increases optical scattering strength in the visible spectrum and emissivity in the mid‐infrared atmospheric transparency window. The restructured silk fibrous film can reduce the average temperature of a substrate, on which the film is coated, by 7.5 °C relative to a nonwoven raw silk fabric during daytime under solar radiation. The results suggest that artificially processed polymeric fiber mats can achieve substantially stronger sunlight rejection than natural silk, by using optical diffusion without Anderson localization. These polymeric mats are useful as sunshades in various applications.