Electrostatic Flocking of Hierarchically Micro/Nanostructured Natural Silk Fibers for Efficient Passive Radiative Cooling

Passive radiative cooling presents great potential to reduce global energy consumption owing to its sustainable features of zero energy consumption and zero CO2 emission. Natural silk fibers exhibit a reflective sheen and a triangular cross-sectional morphology, similar to the attributes observed in the Saharan silver ants' hairs that function to protect the ants from overheating under extremely hot conditions. Here, we demonstrate the facile construction of hair-like arrays of short silk fibers (SSFs) through electroflocking, and the efficient passive cooling performance realized by the enhancement in both the reflectance in the visible to near-infrared range and the emittance in the mid-infrared range. The hairy SSFs flocked on a transparent PDMS film can reduce the temperature of a substrate, on which the film is coated, by 7.6 °C relative to a bare PDMS film when exposed to solar radiation. When flocked on common cotton fabric, the SSFs reduced the temperature of the microenvironment between the fabric and simulated skin by 5.6 °C relative to pristine cotton fabric. Remarkably, the SSF-induced temperature reduction surpassed that achieved with pure silk fabric by 3.6 °C. Such a strategy of electroflocking SSFs offers a simple and robust approach for the large-scale production of highly efficient radiative cooling materials.