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Oil/water separation is of great importance for the treatment of oily wastewater, including immiscible light/heavy oil-water mixtures, oil-in-water or water-in-oil emulsions. Smart surfaces with responsive wettability have received extensive attention especially for controllable oil/water separation. However, traditional smart membranes with a switchable wettability between superhydrophobicity and superhydrophilicity are limited to certain responsive materials and continuous external stimuli, such as pH, electrical field or light irradiation. Herein, a candle soot coated mesh (CSM) with a larger pore size and a candle soot coated PVDF membrane (CSP) with a smaller pore size with underwater superoleophobicity and underoil superhydrophobicity were successfully fabricated, which can be used for on-demand immiscible oil/water mixtures and surfactants-stabilized oil/water emulsion separation, respectively. Without any continuous external stimulus, the wettability of our membranes could be reversibly switched between underwater superoleophobicity and underoil superhydrophobicity simply by drying and washing alternately, thus achieving effective and switchable oil/water separation with excellent separation efficiency. We believe that such smart materials will be promising candidates for use in the removal of oil pollutants in the future.

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The slippage effect at the interface between a confined fluid and the surface around it has a potential application in microfluidic and nanofluidic devices. To investigate the slippage effect of fluid on superhydrophobic surfaces, a series of rheological experiments are carried out for superhydrophobic surfaces coated with carbon nanotube forests (CNTs), which are prepared by the chemical vapor deposition method (CVD). The results show that such surfaces can create certain slippage effects for a fluid flowing over them and that the slip length is in accordance with the theoretical prediction as well as the value measured through other experimental methods. As the same trend is observed for both the theoretical and experimental slip lengths, the theoretical model can be used to optimize the superhydrophobic surfaces for the slippage effect or drag reduction. The superhydrophobic surfaces with considerable slippage effects have broad potential application in micro-and nanofluidic devices or biodevices to solve the fluid resistance problem.