The forces experienced a probe approaching a surface are illustrated here (left). Initially there is a weak replusive force until the probe "jumps" to contact with the surface layer. Further movement towards the surface results in bending of the cantilver due to the stiffness of the surface.

The forces experienced by probes that approach a surface are illustrated to the right. In this case the probe remains stuck to the surface until it can overcome adhesive forces between the tip and the sample.

The total force-distance curve is then:

The force applied after the tip makes contact with the surface can provide a measurement of the stiffness or compliance of the sample while the force required to pull the sample up from the surface is a measure of the adhesion between the tip and the sample. This is illustrated below.

Force distance curves are shown below for a blend of polystyrene (PS) and poly(methyl methacrylate) (PMMA). The inset shows a non-contact AFM image of the surface taken at 110°C. The PS forms domains within the PMMA matrix. This can be confirmed by the force distance measurements that shows that the PMMA behaves elastically (it is still below its Tg) whereas the PS behaves in a viscous manner (note the hysteresis in its force-distance trace).