Friction characteristics of a finger pad sliding on a Si surface coated with organic molecular films, such as octadecyltrichrolosilane (OTS)-SAM and perfluoropolyethers (PFPE), having different film thicknesses were investigated to reveal the effect of film formation state on finger friction. The contact area of the finger pad was also measured using a black ink stamp to elucidate the interfacial phenomena between the finger pad and the surface. The results show that the friction coefficient of the finger decreased with increasing film thickness. This suggests that the friction coefficient was probably affected by surface conditions such as film formation state, even though the difference in film thickness was only a few nanometers. Assuming that the contact area remained unchanged while the finger slid across the surface, a linear relationship between the friction force and the contact area of the finger was observed. The slope of this approximately linear relationship was taken as the shear strength of the interface. The results demonstrated that the shear strength decreased as the film thickness of the coating on the Si surface increased; thus, the interfacial state of the solid was the dominant factor in determining the friction characteristics of the finger pad.

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