3.5.3 Lateral force (friction) mode
If the AFM probe is put in contact with the surface and dragged sideways across it, the cantilever will be sensitive to variations in the lateral force between the probe tip and the sample. Figure 8 illustrates the principle.
With a four-segment photodiode detector, both the bending of the cantilever due to the normal force and the twisting due to the lateral force can be detected. This allows a friction-mode image to be collected simultaneously with the contact-mode topographic image. The friction-mode image yields additional information about the bonding between atoms in the surface being scanned. This opens up the possibility of modifying the probe tip so that it interacts in specific ways with the sample. For instance, if the probe tip is coated with a chemical that binds with another chemical group of particular interest, then it can be used to map that group's distribution on the surface of the sample, through the variation in the lateral force exerted on the tip.