Fig. 3. Characterization of interfacial liquid film behavior and structural deformation of different pillars in the boundary friction process.

(A to C) The interfacial liquid film states and pillar deformation of the BP, NFPF, and PSAN during the sliding in boundary friction. By comparing the adjacent dry and boundary state pillars, the pillar deformation and the interfacial acting force can be achieved. The yellow dashed lines mark the pillar front and rear borders. x_F and x_R represent the deformation at the front and rear sides of pillars in boundary friction. ∆d denotes the change in pillar diameter. The BP and NFPF initially adhere to substrate by capillarity from interfacial nano-thick liquid film and then start to deform and separate from substrate under the shearing action. The separating positions of BP and NFPF are at the rear and front sides of the pillar, respectively. For PSAN, liquid film with smooth round edge and dim color indicates a thick liquid film, and no clear deformation of the pillar occurs under sliding. (D) The interfacial secretion film state of the cricket pad sliding from distal to proximal direction on a glass substrate. The yellow circles denote individual pillars attaching to the glass substrate, and yellow arrows indicate the interference fringes formed between pillars and substrate. Each schematic diagram at the bottom of (A) to (D) corresponds to its friction process. (E and F) The maximum x_F, x_R, and Δd of BP, NFPF-10, NFPF-20, and NFPF-30 in boundary friction with BP being compressed and NFPF being stretched. (G) The interfacial liquid film area changes on different pillars during boundary friction. (H) The interfacial liquid film area proportions on different pillars before utter separation at maximum displacement. Values in (F) and (H) represent the mean, and the error bars represent SD of the measured values (n = 5).