Janus Oil–Water Separation Membranes: Synthesis Approaches, Multifunctional Progress and Prospective Challenges

Rapid industrialization produces massive oily wastewater and frequent marine oil spills, severely damaging ecosystems and human health. Conventional oil–water separation membranes are restricted by single functions, low efficiency, and severe fouling, failing to cope with complex working environments. Featuring asymmetric wettability and multi‐functionality, Janus separation membranes create a new route for high‐efficiency, energy‐saving, and sustainable oil–water treatment. This review systematically summarizes advances in this underreviewed field. It first introduces the origin and development of the Janus concept, then discusses two dominant fabrication methods: layer‐by‐layer self‐assembly and asymmetric chemical functionalization, covering electrospinning, chemical etching, and surface modification, alongside each method's merits and drawbacks. Beyond basic separation, we overview integrated multifunctions including desalination, heavy metal capture, stimuli responsiveness, antibacterial, and self‐cleaning capacities. Critical bottlenecks concerning material stability, fabrication cost and scalable manufacturing are analyzed, followed by prospects of novel substrates, intelligent responsive membranes and eco‐friendly fabrication. This review delivers systematic theoretical guidance and technical references to boost structural optimization, separation performance, and real‐world engineering deployment of Janus oil–water separation membranes.