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Attraction between different contacting surfaces. Adhesion can be described by interfacial work per area, while a measured pull-off force additionally depends on geometry and the elastic response.

At position (1), the AFM probe is far from the surface, with approximately zero force. At (2), attractive interactions bend the AFM cantilever toward the surface; when the attractive force gradient exceeds the cantilever spring constant, the equilibrium becomes unstable and jump-to-contact occurs. Position (3) is on the mechanical-contact branch: changes in imposed height largely bend the AFM cantilever and, for a compliant specimen, indent the sample. This branch can include compressive contact and adhesive contact under tension; contact does not itself imply a positive force at every point. At (4), during withdrawal the tip remains attached and pulls the AFM cantilever downward until the pull-off force is reached and it detaches. Approach and retraction follow different paths because adhesion creates a hysteretic contact. The numbered points identify stages on the source curve, not four extra question parts.

Two useful properties are adhesion and contact elastic modulus. The pull-off force, the most negative force on retraction relative to the noncontact baseline, maps adhesion. Fit the contact part of the force-indentation relation using a suitable contact mechanics model to obtain a reduced elastic modulus; for example, the DMT contact model useswhere $F_{\mathrm{adh}}<0$, $R$ is the calibrated tip radius and $\delta$ is indentation. Converting $E^{\ast }$ to a sample Young's modulus also requires the tip modulus and the two Poisson's ratios. The approach curve alone does not supply a retraction pull-off measurement. The loop area can additionally map dissipated energy; it is not automatically a purely viscous loss.^[ref]