Fig. 3
Friction between calcite and an AFM tip as a function of load in CaCl2 solutions. The selected concentrations are shown in the legend in b. The results are shown at sliding speeds of a 0.2 µm s–1 and b 2 µm s–1, in green for the IS regime and in red for the HS regime. Error bars show the variation in friction over ten friction loops. Tip radius = 100 nm. The insets show the friction coefficient (µ)—calculated as the slope of the friction force vs. normal load—in the LS, IS, and HS regimes at the two selected sliding velocities; the friction coefficient under dry conditions is shown as reference (dashed gray line). The box diagrams illustrate the variation of µ with CaCl2 concentration; the error bars give the standard deviation of µ. The friction coefficient in the LS regime was calculated with experimental data measured with an AFM tip with a smaller stiffness (not shown here). The friction coefficient in the HS regime was calculated above the load Lc and does not include the plateau at the highest loads for simplification. The dependence of friction on velocity shown in Fig. 1 is reflected in the increase of the friction coefficient with velocity (0.2 and 2 µm s–1); note the different scale on the Y-axis
