Dynamic surface force measurement. 2. Friction and the atomic force microscope

The mechanism and geometry of force measurement with the atomic force micro scope are analyzed in detail. The effective spring constant to be used in f orce measurement is given in terms of the cantilever spring constant. Parti cular attention is paid to possible dynamic effects. Theoretical calculatio ns show that inertial effects may be neglected in most regimes, the excepti on being when relatively large colloidal probes are used. Model calculation s of the effects of friction show that it can cause hysteresis in the const ant compliance region and a shift in the zero of separation. Most surprisin g, friction can cause a significant diminution of the measured precontact f orce, and, if it actually pins the surfaces, it can change the sign of the calibration factor for the cantilever deflection, which would cause a preco ntact attraction to appear as a repulsion. Measurements are made of the van der Waals force between a silicon tip and a glass substrate in air. The ev idence for friction and other dynamic effects is discussed. Interferometry is used to characterize the performance of the piezoelectric drive motor an d position detector used in the atomic force microscope. It is shown that h ysteresis in the former, and backlash in the latter, preclude a quantitativ e measurement of friction effects. The experimental data appear to underest imate the van der Waals attraction at high driving velocities, in qualitati ve agreement with the model friction calculations.