Design of a Porous Polydimethylsiloxane Metamaterial for Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) is investigated using porous polydimethylsiloxane (PDMS) with spherical air voids. We combine dependent scattering Monte Carlo simulations in the solar spectral band (0.3-2.5 μm) with full-wave finite element simulations and Nicholson-Ross-Weir effective parameter retrieval in the atmospheric transparency window (8-14 μm) to quantify solar reflectance and thermal emissivity. At high porosity (fv ≈ 0.5), an optimal void radius near 1.4 μm minimizes absorbed solar power while maintaining high thermal emissivity, maximizing net cooling. Extended Maxwell Garnett theory fails for micrometer-scale voids due to multipolar contributions, producing emissivity errors of 0.1-0.15, while retrieval-based effective parameters faithfully reproduce full-wave angular emissivity. The predicted steady-state temperature reduction reaches approximately 6 K at 300 K ambient and 12 K at 315 K. The optimal radius remains stable across ambient temperatures, providing robustness to fabrication variability.