Influence of environmental conditions on shear-force distance control in near-field optical microscopy

In our experiments we show, that a contaminating water film is very importa nt for the shear-force distance control in near-field optical microscopy. T his is demonstrated at the transition between a hydrophilic glass surface a nd a hydrophobic Langmuir-Blodgett film of arachidic acid at different rela tive humidities. This contaminating water film is one, if not the important reason for the damping of an oscillating fiber during surface approach. It is further shown, that the bulk viscosity of water alone cannot be respons ible for the observed damping effect. A thickness dependent viscosity of th is water film is proposed. These observations can also explain, why the she ar-force distance control works on nearly all surfaces at ambient condition s, but fails to work at very low temperatures. (C) 1999 American Institute of Physics. [S0021-8979(99)00924-X].