Fig. 1
Friction between calcite and an AFM tip as a function of velocity in CaCl2 solutions equilibrated with calcite. CaCl2 concentrations are selected as a, f, k 0 mM, b, g, l 1 mM, c, h, m 10 mM, d, i, n 100 mM, and e, j, o 1 M. The friction force in a–e LS regime (0.5–2 nN, σn ≤200 MPa) and k–o HS regime (20–50 nN, σn > 400 MPa) is plotted as a function of speed in log–log scale, while it is plotted in linear-log scale in f–j IS regime (5-20 nN, σn~200–400 MPa). Friction increases with velocity under all conditions. The calculated contact stresses at each load are summarized in Supplementary Table 1. The solid lines show the fits of a–e Eq. (1) and f–j Eq. (2) to the experimental data. The exponent n is shown for each load in a–e. Error bars give the variation in friction across ten friction loops; the variation is often so small that the error bars are smaller than the symbol size, and thus, difficult to see. The blue shades are for LS regime, green for IS regime, and red for HS regime, while the intensity (from light to dark) indicates an increase of load (for each regime). Inset of e and j shows a scanning electron microscopy image of the tip used in LS and IS regimes (with a radius of ~150 nm), and the inset of o shows the tip (with a radius of 100 nm) used in the HS regime. Inset of n shows the velocity-dependent friction force in linear-log scale at two selected loads, 10 and 50 nN, and the shadow qualitatively indicates the reduction of the frictional strength mediated by pressure solution. The arrows in k–o point at the crossover of the velocity-dependent friction force and is the qualitative footprint for pressure solution of calcite
