A Multimodal Biomimetic Structure for All‐Weather Passive Cooling and Building Integration

ABSTRACT Radiative‐evaporative synergistic passive cooling presents a promising sustainable strategy for building thermal management, yet integrating superior optical properties with effective moisture management and structural robustness remains challenging. Herein, a biomimetic hydrogel‐cement bilayer structure inspired by the plant root‐soil synergy mechanism is designed to overcome this limitation. This structure consists of a hydrogel upper layer and a porous cement lower layer. The upper layer handles spectral regulation and evaporative cooling, while the lower layer provides mechanical reinforcement and continuously replenishes water through soil‐like water storage. Irregular Al 2 O 3 nanoparticles create a multiscale porous structure in the hydrogel, which enhances light scattering and enables a remarkable solar reflectance of 97.87% together with a mid‐infrared emissivity of 99%. As a result, the bilayer system attains a sub‐ambient temperature drop of 9°C under intense sunlight. Following rainfall, the cooling enhancement reaches 14°C, and the stored water sustains evaporative cooling for over five days. This work demonstrates a multimodal synergy among optical regulation, moisture management, and interfacial reinforcement, offering a new design paradigm for adaptive radiative‐evaporative cooling materials in buildings.