Robust and flexible polyester fiber membrane with under-liquid dual superlyophobicity for efficient on-demand oil-water separation.

Functional materials with under-liquid dual superlyophobicity have generated a great deal of concern from researchers due to their switchable separation ability oil-water mixtures and emulsions. Conceptually, under-liquid dual superlyophobicity is a Cassie state achievable under-liquid through the synergy of an under-liquid double lyophobic surface and the construction of a highly rough surface. However, obtaining an under-liquid dual superlyophobic surface remains difficult due to its thermodynamic contradiction and complex surface composition. Herein, we successfully prepared a functional coating by modifying the mixture of cellulose nanocrystals (CNCs) and nano-TiO2 with perfluorooctanoic acid (PFOA) via a simple method, then obtained a polyester fiber membrane with under-liquid dual superlyophobicity by roll coating method. The surface wettability of the polyester (PET) membrane was altered, transforming it from the original under-water oleophobic/under-oil superhydrophilic state to the under-water superoleophobic/under-oil superhydrophobic state after coated. The resulting membrane was applied to separate oil and water on-demand. The coated PET membrane exhibited high separation efficiency (>99 %) and high separation flux, effectively separating immiscible oil-water systems as well as oil-in-water and water-in-oil emulsions. The coated PET membrane also demonstrated the ability to perform alternate separation of oil-water mixtures through wetting, washing, and rewetting cycles, with repeated processes up to 10 times without significant reduction in separation efficiency. Furthermore, compared with the previous works, our approach offers a simpler and more convenient method for constructing under-liquid dual superlyophobic surface, making it more suitable for continuous corporate production. This study may provide inspiration for the production and application in large-scale of under-liquid dual superlyophobic membranes.