Daytime passive radiative cooling by ultra emissive bio-inspired polymeric surface

Saharan silver ants can maintain their body temperature below ambient air due to unique triangular shaped hairs that enhance solar reflection and thermal emission through a transparent window that lies in the atmosphere. Applying this thermoregulatory prismatic structure to polydimethylsiloxane (PDMS), highly emsissive in the 8–13 μm spectrum, we present a geometrically modified polymer-based daytime passive radiative cooler. The selective thermal emitter was fabricated based on the optimized prismatic structure from Finite Difference Time Domain (FDTD) simulations. The average emissivity within the 8–13 μm spectrum was enhanced to 0.98 by the gradient refractive index effect, while the average solar reflectivity in the visible and near-infrared spectrum was measured to be 0.95. The net radiative cooling power is estimated to reach 144 W/m², exceeding records of previously reported radiative coolers. Last, in Hong Kong's hot and humid climate, a field test successfully demonstrated cooling by 6.2 °C below the temperature of ambient air corresponding to a net cooling power of 19.7 W/m² in a non-vacuum setup during the peak daytime with shading. This is the largest temperature reduction observed in a tropical region for daytime passive radiative cooling. Our work presents an alternative method to enhance passive thermal emission and may facilitate its world wide application in eco-friendly space cooling.