Design of a monomodal porous metamaterial for passive daytime radiative cooling: Impact of spherical void radius and volume fraction

Passive daytime radiative cooling (PDRC) enables a body exposed to sunlight to dissipate heat through thermal radiation without the need for external energy input. To be effective, such systems must strongly reflect solar radiation across the solar spectral band while simultaneously exhibiting a high thermal emittance within the atmospheric transparency window. In this work, we investigate the cooling performance of a porous metamaterial composed of air voids embedded in a polydimethylsiloxane (PDMS) matrix. Using a combination of Monte Carlo, which takes into account the dependent scattering effect and effective medium theories, such as the Extended Maxwell-Garnett and the Nicholson-Ross-Weir-type retrieval method associated with finite-element electromagnetic simulations, we study the influence of void radius and volume fraction on the spectral properties relevant to PDRC. We identify a range of void radii that optimize performance and demonstrate the existence of an optimal radius that maximizes cooling potential regardless of ambient temperature. These results provide practical design guidelines for the development of efficient PDRC materials and contribute to the broader field of radiative thermal management.