Lateral Forces and Friction

The imaging of surfaces by the SFM in the repulsive mode is based on the dragging of a fine tip across the sample. There are lateral forces between the tip and the sample. At the beginning of a scan the tip sticks to the surface. Later, it will move, but the lever will always feel a force parallel to the surface in addition to the normal force. If the lever is a simple wire, it will bend parallel to the surface. Mate et al.[168] and Erlandsson et al.[176] measured the sideways deflection by interferometry. They detected a variation of the lateral force with the periodicity of the graphite surface.

Figure 4.416b) is a lateral picture of the mica surface. This data was measured by an optical lever SFM, detecting the torsion of a microfabricated cantilever under the influence of friction. The mica periodicity is resolved with a lateral force modulation of $10^-9$ N. The band at the left side with no visible structure is due to the change in the scanning direction. The lateral force changes its sign, hence the width of the band is twice as large as the steady state friction.