Solvent-induced microdroplet scattering interface switching in cellulose membranes enables all-season building thermal management

Rising global building energy consumption underscores the urgent need for sustainable materials capable of passive thermal regulation. Regenerated cellulose, a renewable and biodegradable polymer, presents promising potential for energy-efficient optical systems; however, achieving reversible light modulation with precise solvent responsiveness remains a significant challenge. This study introduces an optically switchable regenerated cellulose membrane fabricated through an ionic liquid-based dissolution-regeneration process. The material incorporates dioctyl phthalate (DBP) to dynamically generate and remove microdroplet scattering domains during solvent exchange. The membrane exhibits exceptional optical properties: a transparent state with 95.2 % transmittance in ethanol/DBP and a scattering state achieving 78.5 % visible reflectance in water. Mechanistic investigations reveal that DBP microdroplets (∼5 μm) form due to their hydrophobic characteristics and long carbon chain structure, creating dynamic light-scattering interfaces. Thermal performance evaluation demonstrates a remarkable 14 °C temperature modulation between transparent and scattering states during summer conditions. In winter operation, the transparent mode increases indoor temperature by 13.9 °C with a heating power density of 269.2 W·m -2 . By integrating biodegradability, mechanical flexibility, and reversible optical switching capabilities, this membrane offers a groundbreaking solution for energy-adaptive window materials and sustainable building thermal management systems.