Friction Effects on Force Measurements with an Atomic Force Microscope

The deflection of an atomic force microscope (AFM) cantilever by a sample that is approached or withdrawn can be used to produce a plot of force versus sample position. These force curves are becoming important for investigating intermolecular forces and surface properties. However, many components of the force curves are not yet understood. Here we show that friction as the tip slides on the surface contributes to the hysteresis in the contact part of the force curve. This sliding arises when the cantilever is mounted at an angle (about 15^° in our microscope) to the surface, which forces the tip to move forward after contact is made during the approach. While the cantilever bends upward, it is bowed forward by the friction. This creates an offset in the contact line which is important to interpreting the curves, in particular when force curves are converted into curves of force versus separation distance, since the contact line is then taken as zero separation. In addition, the sudden onset of the friction at contact can cause a discontinuity in the force curve which sometimes appears as a short range attractive interaction. The friction effect on the contact line is uncoupled from the hysteresis in the noncontact part of the curve, which arises from the viscosity of the medium in which the measurement is made.