Additional file 4: Figure S3. Direct shear test results, showing (A) friction as a function of displacement at a constant velocity of 1.0 µm/s, and (B) constant velocity 0.1 µm/s. Peak friction values are plotted as a function of effective normal stress in the right-hand panels of (A) and (B), which show fitted peak friction values of 0.40 (A) and 0.44 (B) as the slopes to linear fits.