Study of shear force between glass microprobe and mica surface under controlled humidity

Resonance characteristics of a glass micropipette near a mica surface have been investigated as functions of the tip-surface distance, D, and of the ambient humidity, H. With decreasing D, the resonance frequency of the tip increases, while its resonance amplitude decreases. The resonance curve is almost symmetric except in the close vicinity of D=0, at which the oscillation is damped completely. The effective length, D-0, of the tip-surface interaction is independent of the dither amplitude. With increasing H, D-0 decreases gradually, exhibiting a rather sharp drop around H =40%. These results indicate that, for large D where the direct contact does not occur, some noncontact force between the tip and the surface is operative, and that this force is sensitive to the presence of a water layer on the surface. (C) 1997 American Institute of Physics.