Drag reduction technology has a promising application in marine fields and has drawn much interest in scientific fields. Superhydrophobic surface has been proven to be effective in drag reduction due to thin film of gas adsorbed on surface because of its low friction drag and large slip length. Here, macroscopic and stable gas film was observed when water flowed over superhydrophobic surface with step without additional gas injection under laminar flow and turbulent flow. Superhydrophobic surface was prepared with contact angle more than 150° and roll-off angle nearly 0°. Macroscopic gas film could maintain under laminar flow and turbulent flow and keep up to 80% after 1 h water flowing with optimized parameters of step, showing different morphological deformations under different velocities and Reynolds numbers. Compared with untreated hydrophilic surface, superhydrophobic surface with step exhibited good drag reduction performance with maximum drag reduction rate 20% under laminar flow and turbulent flow, after optimizing of height of step and distance between steps. Mechanisms of gas film drag reduction were the ultra-low skin friction drag force between liquid–gas interface, large slip length on liquid–gas interface, and flexible gas film surface acted like compliant wall. Gas film of millimeter scale was much larger than thickness of boundary layer and reduced turbulence intensity near wall. This work provides a new way to obtain macroscopic gas film and analyze liquid–gas interface.

Smart surfaces with controllable friction have generated considerable attention lately. However, most composites prepared with traditional fillers cannot achieve “real-time” friction conversion. Herein, a new smart surface was designed to achieve different friction coefficients (0.65 and 0.12). Different coefficients of friction were reversibly and precisely controlled via heating. Via friction and heating, 1H,1H,2H,2H-perfluorohexyl hexadecane (PHHD), a kind of phase-change material—paraffin wax—was released from the microcapsules, and a stable and complete film was formed. It changed the interface from “solid-solid” to “solid-liquid” in a dry friction state. The composite contains microcapsules that prevent phase separation between PHHD and matrix, which enables the composite to have a long service time and switchable friction performance. In addition, this composite can maintain its extraordinary ability even in harsh environments like UV irradiation. By demonstrating switchable friction based on changes in the interactions between contact interfaces, this work provides a new principle for designing smart tribological composites.