Supplemental Material
Fluorescence intensity calibration method (FIG. S1, S2), the power spectral density of the surface topography (FIG. S3), sphere AFM topography before and after contact (FIG. S4), shear stress as a function of normal stress (FIG. S5), normal contact stiffness calculation with and without plasticity (FIG. S6), and mechanical properties of materials (TABLE S1).