Maintaining large-scale gas layer by creating wettability difference on surfaces under water

Superhydrophobic surfaces with micro- and nano-scale structures are conducible to maintaining a gas layer where prominent slippage effect exists. It has been demonstrated that the drag reduction of superhydrophobic surface increases with growing the fraction of the gas-water interface and the rising of the thickness of gas layer. Whereas a large thick gas layer on the superhydrophobic surface collapses easily under tangential water flow. Here, we present a new method to maintain large-scale gas layer by creating hydrophilic patterns at the superhydrophobic surface, on which the binding force of air on the solid surface can be caused by wettability difference. Through testing the states of gas layer trapped on surfaces with wettability differences equal to 54.8, 84.7, 103.6 and 144.0 in apparent contact angle, respectively, the conditions of maintaining gas layer are mainly considered. We demonstrate that the critical velocity, over which the gas layer begins to collapse under the tangential water flow, is positively correlated with the thickness of the gas layer and the wettability difference between the superhydrophobic area and hydrophilic area, however, this is negatively correlated with the width of the gas layer in the crosswise direction. It is noteworthy that even a centimeter-scale gas layer can be kept steady in ～0.9 m/s through this method. Furthermore, an obvious slip velocity up to ～25% of bulk velocity is observed at the gas-water interface, through measuring the velocity profile above the 0.6 cm-long, 0.5 cm-wide and 0.15 cm-thick gas layer by using the PIV technology. We anticipate that this novel method of gas entrapment under water will effectively widen the applications of superhydrophobic surfaces for drag reduction.