Stable air plastron on butterfly-inspired superhydrophobic surfaces for drag reduction

Superhydrophobic surfaces have been widely applied to drag reduction in under water vehicles due to their excellent water repellency and capability of maintaining an air layer. In nature, many animals and plants have superhydrophobic surfaces to repel water and reduce resistance, among which butterfly wings are good examples. Here, a biomimetic butterfly superhydrophobic surface (BBSS) was prepared on aluminum alloy by repeated laser melting and hydrophobic treatment. The maximum contact angle of the BBSS was 157.5°and the rolling angle was only 0.8°. The BBSS could entrap a layer of air underwater, which remained intact throughout the process of water erosion (with a flow velocity of 4.48 m/s) for ∼3 h, demonstrating the high stability of the air layer. Due to the presence of the air layer, the solid-liquid interface was transformed into the air-liquid interface, thereby reducing the frictional resistance. The BBSS exhibited a maximum drag reduction rate of 30.12% at the flow velocity of 2.11 m/s. At a higher flow velocity of 4.48 m/s, the drag reduction rate of the BBSS still reached 15.13%. Moreover, after continuing water erosion for ∼5 h at a flow velocity of 4.48 m/s, the BBSS surface still had drag reduction effect. The results demonstrated that the BBSS has a strong capability to maintain a stable air layer underwater, which provides a simple and efficient way to achieve a significant drag reduction.