Figure 4.

Boundary friction enhancing of rim self‐sucking effect. a) On concave pillar, liquid film area remains unchanged during evaporating as liquid in cavity continuously flows to rim area. The cavity bottom is gradually pulled to substrate and forms rim self‐sucking effect. Gap distance at rim of concave pillar d _M ^CP is ≈50 nm thinner than d _M ^SP on smooth pillar. b) On bioinspired hierarchical concave pillar surface, liquid self‐splitting effect could induce multiply rim self‐sucking simultaneously. c) Concave pillar surfaces demonstrate stronger boundary friction than smooth surface. With ratio of rim area to pillar area K gradually decreasing, stronger self‐sucking effect forms and generates stronger boundary friction. No significant difference shows among smooth surface and pillar surface with K = 1, 0.9 under one‐way ANOVA with p = 0.8. While significant difference shows for surface with K = 0.9–0.5 under one‐way ANOVA with p < 0.05 (sample size n = 10).