Force curves plot the distance of the scanner movement (horizontal axis) vs. the cantilever deflection (vertical axis) and are used to measure the vertical force of the tip on the surface. In contact mode this can be used to examine the attractive, repulsive, and adhesive interactions between the tip and the sample as well as to set the imaging force.
Force curves are specific to the system they are taken from and are often complex. The simplest case results from an AFM system in a vacuum. The curve begins with the tip not touching the sample so the cantilever is undeflected and the scanner is fully retracted. The cantilever comes close enough to the sample to experience an attractive van der Waals force as the scanner extends and suddenly bends toward the surface. The cantilever deflects away from the surface until the scanner reaches full extension and begins to retract. At this point the cantilever deflection retraces the same curve as the scanner pulls the tip away from the surface.
It is possible for a monolayer of water to create a capillary force on the tip as the scanner pulls away from the surface creating a difference in the retracting curve. The point at which the tip springs free is referred to as the snap-back point. Multiple snap-back points may occur if a lubrication layer is present in addition to the water layer. Viscosity and thickness of these layers determine the amplitude and position of these points.
The slope of the linear region of a force curve is related to the elastic modulus of the system. The slope of the curve mostly reflects the spring constant of the cantilever when the cantilever is softer than the sample. The curve reflects elastic properties of the sample when the cantilever is stiffer than the sample.