Drag reduction and plastrons stability of sandpaper-based superhydrophobic surfaces in turbulent flows

A superhydrophobic surface (SHS) produces significant drag reduction in turbulent flows by supporting a thin layer of gas (plastron) on the surface. Recently, we developed a simple method to fabricate SHS by sprayed-coating a layer of hydrophobic silica nano-particles over sandpapers. In this work, we reported the drag reduction and plastrons stability of the sandpaper-based SHS in a turbulent channel flow facility. The flow was generated by a 3-horsepower centrifugal pump and the internal dimensions of the channel were 1016 mm × 50 mm × 6.4 mm (length × width × height). The mean flow velocity varied from 0.5 to 5.5 m/s. The Reynolds number, calculated based on the channel height and mean flow velocity, varied from 3,200 to 64,000. The 960 mm long SHS was installed at the bottom of the channel. The SHS had a root-mean-square roughness height varying from 4 to 70 µm by using sandpaper of different grit sizes from 60 to 1500. The amount of drag reduction of SHS was measured based on the pressure drops in the fully developed region of the channel, as well as by high-resolution velocity measurements near the SHS. In the meanwhile, the gas layer on the SHS was measured by reflection-based microscopy imaging. The results showed that the SHS consisted initially of a continuous air layer at low Reynolds number flows and produced a large drag reduction. However, as increasing Reynolds number, the air layer became isolated air pockets and the drag reduction by SHS diminishes.