The effect of humidity on probe-sample interactions in near-field scanning optical microscopy

The shear force damping between a tapered near-field probe and a well-cleaned glass sample depends on the humidity. A slow decay region, up to few hundred Å, appeared in the onset of the damping curves as the humidity increased. This humidity-dependent interaction force is of viscous type and ∼0.1 nN. The frictional coefficient increases faster than the estimation from Newton’s law of viscosity as the probe and sample distance decreases. This is attributed to the enhancement of viscosity in thin water layer.