Resonance frequency shifts caused by the friction of a drop of water in air: An approach to estimate shear forces in scanning probe microscopies

In scanning near-field optical microscopy, scanning ion conductance microsc opy and for localized electrochemical deposition out of micropipettes, the detection of shear forces between the tip and sample is one of the most com mon methods of distance control. Here, pulled micropipettes were utilized t o form an evaporating drop of water whose frictional force in air causes a specific resonance shift of the tip vibration. This resonance shift and the amplitude at the resonance were investigated with regard to their dependen ce on the drop diameter. In order to calculate the friction, the tip is app roximated as a damped harmonic oscillator. The typical range of the shear f orces in scanning probe microscopies is estimated to be between 1 pN and 0. 1 nN. (C) 2001 American Institute of Physics.