Shear force interaction in the viscous damping regime studied at 100 pN force resolution

We report a very good force resolution for tuning fork based shear force mi croscopy as used for feedback regulation in scanning near-field optical mic rocopy (NSOM). The sensitivity and dynamics of fiber tips attached to 100 k Hz tuning forks are investigated both experimentally and theoretically appl ying a finite element analysis. Operating the tuning fork at vibration ampl itudes smaller than 10 nm allows to discriminate between viscous damping du e to capillary wetting, and fiber bending upon tip-sample approach to hydro philic sample surfaces indicating the direct transition from "noncontact op eration" (pure viscous damping due to contamination layer) down to tip-samp le contact. Viscous damping manifests in frequency shifts of less than 50 m Hz, as deduced from resonance curves recorded under feedback control. For r elative amplitude changes of less than 0.5% the viscous damping force actin g lateral on the fiber tip is calculated to similar to 100 pN using the fin ite element method. This detection limit proves that tuning fork based shea r force control is superior to other feedback mechanisms employed in NSOM. (C) 2000 American Institute of Physics. [S0021-8979(00)02106-X].