Robust Radiative Cooling in Functionalizable Silica Microsphere Paints

Disordered coatings based on silica microspheres provide a scalable and robust platform for passive daytime radiative cooling. While particle-size optimization is often considered critical for enhancing solar scattering, the role of microsphere diameter once a coating operates in the multiple-scattering regime remains unclear. Here, we characterize the radiative cooling performance of disordered, multiply scattering photonic glass coatings with diameters ranging from 2 to 8 μm, deposited on a highly reflective substrate. Despite measurable differences in microscopic scattering properties, both the spectral radiative response and the net cooling performance are robust to variations in particle diameter within the investigated range. Outdoor thermal measurements reveal nearly identical steady-state temperature reductions across the full size range. These results identify this robustness as a property of the complete coating-substrate architecture: in the solar range, multiple scattering and the reflective substrate act together to maintain high reflectance, while in the mid-infrared range, the response is dominated by the intrinsic Si-O absorption of the silica layer. This robustness enables microsphere size to be selected based on surface chemistry or processing constraints without compromising cooling performance.