Self‐Responsive Janus Hydrogel with Alternation of Day and Night for Highly Efficient Solar Evaporation

Solar‐driven interfacial evaporation technology offers a promising method for sustainable freshwater production. Hydrogels have emerged as promising materials for interfacial evaporation, as they possess the capability to activate water molecules and lower the enthalpy required for evaporation. However, monolithic hydrogels fail to meet the diverse water content requirements in different evaporation regions. Herein, a thermoresponsive hydrogel (CSAm) capable of spontaneously forming Janus structures under sunlight illumination is designed. CSAm exhibits reversible solar‐induced wetting transition, enabling efficient evaporation with asymmetric wetting properties during the day and self‐cleaning of salts with hydrophilic wetting properties during the night. This transition of Janus structures allows for the regulation of water content distribution within the evaporator, meeting the diverse water content requirements of different evaporation regions. CSAm is composed of a thermoresponsive polymer, poly( N , N ‐diethylacrylamide), and a hydrophilic polymer, polyacrylamide, cross‐linked to form a scaffold structure. The uniform dispersion of photothermal material (CuS) within the gel scaffold is achieved using lignosulfonates. This study investigates the Janus structure characteristics, photo/thermoresponsive properties, salt resistance, and evaporation performance. Under 1 sun illumination (1 kW m −2 ), CSAm exhibits a high evaporation rate of 2.67 kg m −2 h −1 and demonstrates long‐term stability in seawater desalination.