The superhydrophilic self-cleaning surface can perfectly deal with oil pollution, which cannot be realized by the superhydrophobic surface. This research is designed to study the mechanism of wetting behavior of superhydrophilic coating with different function groups and guide to design a stable self-cleaning surface. We prepare several hydrophilic coatings including nonionic, ionic, and zwitterionic coatings to investigate their self-cleaning performance underwater when they have been polluted by oil in the dry state. The chemical composition, surface roughness, static and dynamic wettability, underwater oil adhesive force, and swelling degree of the coatings are studied to explore their oil dewetting mechanism. The results indicate that the wettability of the coating to water and oil is the key factor to determine the self-cleaning performance. The smooth 3-sulfopropyl methacrylate potassium salt (SA) anionic coating shows the best self-cleaning performance even when polluted by heavy crude oil in the dry state in air. It is also found that in the dry state, the rough hydrophilic anionic surface will lock up the oil in the structures and then lose its self-cleaning ability underwater, whereas the oil droplet can detach from the smooth coating surface quickly. Meanwhile, the superhydrophilic and underwater superoleophobic SA anionic surfaces also exhibit excellent anti-fogging and oil−water separation performance.

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plants in nature, but also is closely related to human production and life. [1,2] Superwettable surfaces have aroused much interest because of their wide applications in many fields. [3-6] For example, the superhydrophobic surfaces with water contact angle (WCA) higher than 150° and water sliding angle (WSA) lower than 10° are widely used in drag reduction, anticorrosive and anti-icing. [7-9] On the other hand, various other functions, such as antifogging, antifouling, self-cleaning and even heat transfer are expected to be realized on superhydrophilic surfaces with WCA lower than 5°. [10-13] In addition to water, oil is another kind of important liquid whose surface tension (30 mN m −1) is much lower than that of water (72 mN m −1). [14] Super-oil-wettable surface (superoleophilic and superoleophobic surfaces) also has tremendous potential applications due to its extremely wetting and anti-wetting properties. [15-17] Superamphiphobic material is the most ideal self-cleaning material due to its non-wetting property to oil and water, but excessive structural requirements and instability greatly limit its further development. [18-20] The commonly used superhydrophilic or superhydrophobic self-cleaning materials can deal with water and solid pollutants well, but they are incapable of resisting oil pollution because of their oleophilicity. A feasible method is to use the underwater superoleophobicity for a superhydrophilic surface to be pre-wetted with water to avoid contamination or be put into the water to realize decontamination. [21-23] But it is unrealistic to pre-wet the material in actual conditions and most materials are used in air, which puts high demands on the decontamination ability of the material. According to our previous study, [24] for the two superoleophilic surfaces with the same chemical composition after oil contamination in a dry state, the smooth surface can quickly desorb oil droplet and achieve self-cleaning after being placed underwater, while oil droplet is locked in the micro-structure of rough surface and difficult to desorb. This indicates that rough structure is not conducive to the self-cleaning of oil pollution. But the application of rough surfaces is far beyond smooth surfaces, especially in wettability. [25-28] Therefore, it is necessary to figure out whether oleophilic/superoleophilic rough surface can achieve a similar oil dewetting process from air to water to Responsive smart surfaces with controllable wettability have a high potential application in self-cleaning and oil-water separation. In this work, a new and simple method is reported to fabricate a pH-responsive superwettability surface. And the self-cleaning performance from air to water after oil contamination in a dry state is studied. Wettability regulation can be achieved by controlling the ratio of carboxyl groups with protonation or deprotonation state in acid or basic solutions. Rough surface with 40% carboxyl content (X COOH = 0.4) shows the most significant response. The surface is superoleophilic underwater aft...

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