ABSTRACT
Approach and withdrawal curves can be divided roughly into three regions: the contact line, the non-contact region and the zero line (Fig. 4). The zero line is obtained when the tip is far from the sample and the cantilever deflection is close to zero (when working in liquid, this line gives information on the viscosity of the liquid [74]). When the sample is pressed against the tip, the corresponding cantilever deflection plot is called the contact line and this line can provide information on sample stiffness. The most interesting regions of the force curve are two non-contact regions, containing the jump-to-contact and the jump-off-contact. The non-contact region in the approach curve gives information about attractive (van der Waals or Coulomb force) or repulsive forces (van der Waals in some liquids, double-layer, hydration and steric force) before contact; this discontinuity occurs when the gradient of the tip-sample force exceeds the spring constant of the cantilever (pull-on force). The non-contact region in the withdrawal curve contains the jump-off-contact, a discontinuity that occurs when the cantilever's spring constant is greater than the gradient of the tip-sample adhesion forces (pulloff force). A convenient way to measure forces with precision is to convert them into deflections of a spring, according to Hooke's law:
where the cantilever deflection Sc is determined by the acting force F and the spring constant of the cantilever, kc.
