Frictional force microscopic observation of anisotropy at corrugated CaSO4(001) surface

Abstract

Measurement of frictional coefficients with frictional force microscopy (FFM) at CaSO₄(001) showed marked anisotropy due to a ridge-and-valley structure of the surface, stronger friction being observed upon scanning the FFM probe across the ridges. The anisotropy is explained by counting the numbers of top-most O atoms touching the probe surface. The probe interacts with twice as many surface atoms in the valleys as on the ridges. Upon scanning the probe across the ridges, it experiences stronger and weaker interactions alternately, more work being required to put the probe from the valleys to the mountaintops. Less work is needed to scan the probe along a ridge or a valley due to more stable interaction potentials. A specifically low friction along the [210] direction was also explained in the same way. Friction along the ridges did not show dependence upon relative humidity (RH), while the friction across the ridges initially increased, and then, decreased with RH. It is likely that water molecules specifically adsorb at valley sites forming stripes at the initial stage of adsorption.