Fig. 1.

(A) Schematic of the principle of SFA used for measuring normal load, L, and friction force, f, between two surfaces in contact with area A under back and forth shearing motion of peak-to-peak amplitude D ∼100 μm and driving velocity, v. The friction force, f, is measured as f = K(Δx − ΔD), where Δx and ΔD are the displacement of the drive and the stage, respectively, and K is the stiffness of the measuring spring. (B) Evolution of friction forces with time measured between two cartilage surfaces in synovial fluid at a driving speed of v = 10 μm/s. Right after loading and applying shear motion, the friction force increases rapidly and evolves to stick-slip or irregular stick-slip sliding motion (B Insets). After sliding for 3 min, the friction force gradually decreases to a steady-state value, and sliding phase becomes smooth. Four distinct sliding behaviors are observed depending on the sliding conditions: (C) regular stick-slip, (D) irregular stick-slip, (E) smooth sliding with stiction, and (F) purely smooth sliding.