Fig. 2. Characterization of structures and wet friction properties of BP, PSAN, and NFPF surfaces.

(A) Scanning electron microscopy images of different types of pillars with the diameter d, channel width w, and height h of 100, 25, and 10 μm, respectively. Nanofibers in NFPF and PSAN with a diameter of 400 nm. (B) Shear stress for different surfaces during successive friction tests, and their interfacial liquid film states at boundary friction are compared. Interference fringes are only observed on BP and NFPF surfaces. The air gap appears at the rear side of BP and the front side of NFPF during sliding. (C) Boundary frictional shear stress for different pillar surfaces on various hydrophilic substrates. (D) Boundary frictional shear stress for different pillar surfaces on various roughness substrates. BP/NFPF-number denotes BP/NFPF of the specific height. All friction tests were conducted without external normal force and with an area of ~1 cm². PTFE, polytetrafluoroethylene; RMS, root mean square. Values in (C) and (D) represent the mean, and the error bars represent SD of the measured values (n = 5).