Underwater superoleophobicity of a superhydrophilic surface with unexpected drag reduction driven by electrochemical water splitting

In a fluid transport system, fouling and drag force are key issues because they cause additional expenditure for cleaning and fuel consumption, respectively. Various surface treatments, especially wettability control, have been explored to solve the related problems. However, surface modification has inherent limitations because it has conflicting effects on the anti-fouling and drag force. A superhydrophilic surface has an excellent anti-fouling property underwater because of its excellent oleophobicity, but it also causes drag enhancement due to its high affinity towards water. On the other hand, the superhydrophobic surface shows a drag-reducing effect due to the formation of an air layer underwater, but at the same time, it is easily contaminated by oil due to its high oleophilicity. Herein, we suggest a novel method to overcome these conflicting effects for a superhydrophilic surface for anti-fouling and drag enhancement by employing electrochemical gas generation. Due to its large effective surface area and underwater superaerophobicity, the superhydrophilic surface is ideal for use as an electrocatalyst for electrochemical water splitting (EWS). The small and fine air bubbles (similar to 30 mu m sizes) generated on a superhydrophilic electrode surface form an air layer, which lowers the fluid density near the surface and induces a drag reduction efficiency of similar to 26.5% compared to the flat surface. This value is comparable to the superhydrophobic surface, which has a high drag reduction efficiency. The current study presents an original idea that combines the oleophobicity and superaerophobicity of a superhydrophilic surface for realizing EWS-bubble induced anti-fouling and drag reduction effects simultaneously, which has not been reported before.