Biodegradable Janus Membrane Featuring Asymmetric Wettability and Pore Structures for Efficient and Salt-Resistant Solar Desalination

While solar-driven interfacial evaporation holds great promise for addressing freshwater scarcity, its widespread reliance on non-degradable synthetic polymers may raise concerns regarding secondary microplastic pollution. Furthermore, balancing continuous water supply, thermal localization, and anti-scaling remains a fundamental interfacial challenge. Herein, we report a biodegradable Janus photothermal evaporator composed of a hydrophilic carboxylated carbon nanotubes (CCNTs)-modified viscose layer and a hydrophobic electrospun polylactic acid (PLA) nanofibrous layer. Benefiting from its asymmetric wettability and distinct pore structures across the bilayer interface, the Janus architecture exhibits directional water transport behavior. This structural design generates a capillary pressure difference that enables self-pumping, unidirectional water transport. Consequently, the directional fluid delivery helps maintain water replenishment at the evaporation interface, improving interfacial heating and reducing local salt accumulation during evaporation. When assembled into a 3D conical geometry, the evaporator delivers an apparent evaporation rate of 1.89 kg·m-2·h-1 with solar-to-vapor efficiency of 99.55% under one-sun illumination, while maintaining stable evaporation performance over 20 cycles in 3.5 wt % NaCl solution. The PLA/viscose membrane exhibits sustainability, showing a degradation rate of over 70% within 35 days under composting conditions. This work provides insight into the role of asymmetric wetting interfaces and offers a biodegradable design strategy for solar interfacial evaporation systems.