Wettability step electrode to generate millimeter-scale gas–liquid interface for drag reduction

Superhydrophobic surfaces can seal the gas–liquid interface (GLI) under water to produce the drag reduction effect. Enhancing the stability and slip length of the GLI is an important issue in this context. Herein, we fabricate wettability step electrodes (WSEs) by creating an array of millimeter-scale circular superhydrophobic regions on a hydrophilic graphite plate by using an economical and efficient mask spraying method. When the WSE was electrified as an anode, the oxygen produced by the electrolytic reaction was preferentially precipitated in the superhydrophobic regions and ultimately formed an array of millimeter-scale GLIs. The evolution process of this GLI can be divided into a spreading stage and a growth stage. The results of experiments revealed that the spreading duration of the GLI increased with the diameter of the superhydrophobic regions (D) and decreased with the spacing between adjacent superhydrophobic regions (L). During the growth stage, the height of the GLI decreased with the ratio D/(L + D) and increased over the duration of electrification according to a 1/3 power-law relationship. Finally, we measured the slip characteristic on a single millimeter-scale GLI by particle image velocimetry. The result showed that the effective slip length of the GLI with a streamwise length of 2 mm can exceed 100 μm, thus confirming the potential of the millimeter-scale GLI for drag reduction.