Heterogeneous wettability membrane for efficient demulsification and separation of oil-in-water emulsions

Single super-wetting membranes have been used to treat oily wastewater, but most membranes lack demulsification ability and often exhibit poor separation performance. Herein, we fabricate a desert beetle-like heterogeneous wettability membrane with a superhydrophilic fiber substrate and weakly hydrophilic discontinuous protrusions for demulsification and separation of oil-in-water emulsions. The unbalanced force formed by the difference in hydrophilicity between the substrate and protrusions promotes the demulsification of the emulsion. Additionally, the protrusions capture and gather dispersed oil droplets due to Laplace pressure, forming a layered oil slick. This synergistic effect overcomes the limitation of traditional size-sieving mechanism, enhances the separation performance, and avoids membrane fouling. The heterogeneous wettability membrane demonstrates a 99.8 % gravity-driven oil rejection for various oil-in-water emulsions with a permeability of 2600 L·m−2·h−1. It can also achieve effective separation of high viscosity crude oil emulsions without contamination. Furthermore, molecular dynamic simulations reveal that the heterogeneous wettability structure of the membrane surface promotes the demulsification and separation of emulsified oil droplets. And the presence of two hydration layers is found to be the main reason for the great antifouling property of the membrane. This study provides valuable insights for the design of separation membranes for oil-in-water emulsions and the advancement of separation technology.