Hierarchically Structured Fluorine-Free Superhydrophobic Composite Films for Efficient and Durable Passive Daytime Radiative Cooling

Abstract Passive daytime radiative cooling (PDRC) offers continuous cooling without external energy input, yet conventional radiative cooling materials often suffer from environmental contamination and performance degradation, while fluorinated systems raise ecological concerns. Here, we report a fluorine-free, superhydrophobic TPU/PDMS@HGM radiative cooling film with a hierarchical micro/nanostructure, fabricated via a scalable electrostatic spraying approach. Polydimethylsiloxane (PDMS)-modified hollow glass microspheres (HGM) were synergistically incorporated to enhance both solar scattering and interfacial wettability. The densely packed PDMS@HGM network introduces multiple photon backscattering pathways, effectively suppressing solar absorption, while the intrinsic vibrational modes of PDMS within the atmospheric window promote mid-infrared thermal emission. The resulting film achieves an average solar reflectance of 96.46% and an atmospheric-window emissivity of 91.78%, delivering a temperature reduction of 12.58 °C in outdoor tests relative to conventional polyester canvas. Simultaneously, the hierarchical rough surface maintains a stable Cassie–Baxter wetting state, with a water contact angle of 152.8° and a sliding angle of 6.35°, while sustaining superhydrophobicity after acid–alkali immersion, water washing, and dirt-rinsing treatments. This work demonstrates a robust strategy for designing fluorine-free flexible radiative cooling materials that combine high solar reflectance, efficient thermal radiation, and exceptional environmental durability, offering promising potential for wearable thermal management, outdoor shading, and smart textile applications.