Directional transport enables oil-water separation: Mechanisms, fabrication, and performance evaluation.

Oily wastewater comes from many sources and has complex compositions. Traditional oil-water separation methods often suffer from high energy consumption or low efficiency. Although biomimetic directional liquid transport materials are developing rapidly, a systematic review focusing on their application in oil-water separation is still lacking. To address this gap, this review provides a comprehensive review of recent advances in directional liquid transport materials for oil-water separation. First, typical organisms in nature that have directional liquid transport functions are introduced. Three types of directional transport mechanisms and their physical principles are summarized: wettability-gradient driven, structure-gradient driven, and synergy-gradient driven. Second, the common preparation methods for each of the three driving mechanisms are classified. The advantages and disadvantages of these methods are compared. Third, the key performance evaluation indicators are outlined. These include transport rate, directional transport distance, critical breakthrough pressure, and the directional transport index. Next, the current application status of these materials in oil-water separation is highlighted. Finally, the main challenges and future development directions are discussed. This review aims to serve as a reference for the design, preparation, and practical application of high-performance directional transport materials for oil-water separation.