Search

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. [ABSTRACT FROM AUTHOR]

The gas–liquid interface (GLI) over superhydrophobic surfaces (SHSs), where the flow slips, is the key to reduce frictional drag in underwater applications. Many-body dissipative particle dynamics simulations are used to explore the slip behavior of a shear flow over a rectangular grooved SHS, and a flat GLI is obtained by tuning the contact angle of the GLI. Due to the slip, the normal profiles of the local velocity, which are perpendicular to the GLI, are curved and shifted away from the linear form near the GLI. Then, a polynomial function is proposed to fit the velocity profile to extract the local shear rate and calculate the slip length. Based on this fitting method, a hybrid slip boundary condition is derived for both longitudinal and transverse flows. That is, the shear stress and slip length are finite near the groove edge, and the stress is nearly zero and the slip length is infinite in the center region of the GLI. This new hybrid slip boundary condition not only explains the inconsistent slip conditions reported in the literature under different groove length scales, but also unifies the existing exclusive slip assumptions. [ABSTRACT FROM AUTHOR]