Oil leakage and the discharge of oil/water mixtures by domestic and industrial consumers have caused not only severe environmental pollution and a threat to all species in the ecosystem but also a huge waste of precious resources. Therefore, the separation of oil/water mixtures, especially stable emulsion, has become an urgent global issue. Recently, materials containing a special wettability feature for oil and water have drawn immense attention because of their potential applications for oil/water separation application. In this paper, we systematically summarize the fundamental theories, separation mechanism, design strategies, and recent developments in materials with special wettability for separating stratified and emulsified oil/water mixtures. The related wetting theories that unveil the physical underlying mechanism of the oil/water separation mechanism are proposed, and the practical design criteria for oil/water separation materials are provided. Guided by the fundamental design criteria, various porous materials with special wettability characteristics, including those which are superhydrophilic/underwater superoleophobic, superhydrophobic/superoleophilic, and superhydrophilic/in-air superoleophobic, are systemically analyzed. These superwetting materials are widely employed to separate oil/water mixtures: from stratified oil/water to emulsified ones. In addition, the materials that implement the demulsification of emulsified oil/water mixtures via the ingenious design of the multiscale surface morphology and construction of special wettability are also discussed. In each section, we introduce the design ideas, base materials, preparation methods, and representative works in detail. Finally, the conclusions and challenges for the oil/water separation research field are discussed in depth.

Although artificial superhydrophobic materials have extensive and significant applications in antifouling, self-cleaning, anti-icing, fluid transport, oil/water separation, and so forth, the poor robustness of these surfaces has always been a bottleneck for their development in practical industrial applications. Here, we report a facile, economical, efficient, and versatile strategy to prepare environmentally friendly, mechanically robust, and transparent superhydrophobic surfaces by combining adhesive and hydrophobic paint, which is applicable for both hard and soft substrates. The coated substrates exhibit excellent superhydrophobic property and ultralow adhesion with water (contact angle ≈ 160° and sliding angle <2°). Additionally, the coated surface maintained its superhydrophobicity even after 325 sandpaper abrasion cycles, showing remarkable mechanical robustness. Furthermore, the coated surfaces were applied to separate oil/water mixtures because of their unique characteristics of being simultaneously superhydrophobic and superoleophilic. In addition, it is believed that this fabrication method is significant, promising, and feasible for mass production of superhydrophobic surfaces for industrial applications.

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This research work focuses on the wetting behavior and directional motion of water droplet on the nanocone surface through molecular dynamics simulation approach. A total of five nanocones having different apex angle were utilized to observe the wetting characteristics. In addition, water droplets of different sizes based on containing number of molecules were also considered in this study. The results show that the water droplet spontaneously travels from the tip of the nanocone toward its larger diameter direction in all cases. The average velocity of water droplet on the nanocone surface with smaller apex angle was found to be higher than that with larger apex angle. Moreover, it was found that the energy parameter of the nanocone surface and the size of water droplet have a significant effect on the wetting characteristics and velocity of droplet. Furthermore, three different equilibrium states of the water droplet on the nanocone surface were observed; unwet, partially wet, and fully wet. The velocity of the droplet was comparatively higher in the fully wet state as compared to the partially wet equilibrium state.

Although various superhydrophobic/superoleophilic porous materials have been developed and successfully applied to separate water-in-oil emulsions through sizesieving mechanism, the separation performance is restricted by their nanoscale pore size severely. In this study, the wettability of underoil water on fumed silica was experimentally observed and underlying mechanism was investigated by carrying out theoretical analysis and molecular dynamic (MD) simulations. Further, we present a novel, facile and inexpensive technique to fabricate underoil superhydrophilic metal felt with micro pores for separation of water-in-oil emulsions by using SiO2 nanoparticles (NPs) as building blocks. The as-prepared underoil superhydrophilic coating is closed-packed and ultrathin (the thickness is approximate hundreds of nanometers), as well as capable of being coated on metal felt with complex structures without blocking its pores. The as-prepared metal felt could adsorb water droplets directly from oil, which endowed it with the ability to separate both surfactant-free and surfactant-stabilized water-in-oil emulsions with high separation efficiency up to 99.7% even though its pore size is larger than that of the emulsified droplet. The filtration flux for separation of span 80-stabilized emulsion is up to ~4000 L • m −2 • h −1 . Its separation performance is better than most of the other traditional membranes and superwettable materials used for separation of water-in-oil emulsions. Moreover, the as-prepared metal felt retained outstanding separation performance even after 30 cycles of use, which demonstrated its excellent reusability and durability. Additionally, the distinctive wettability of underoil superhydrophilicity endued coated metal felt with superior anti-fouling properties towards crude oil. Overall, this study not only provides a new perspective on separating water-in-oil emulsion, but also gives a universal approach to develop special wettability materials.