ABSTRACT

Figure 4A includes the calculated van der Waals attraction prior to contact, F(h) = AR/\2(h —d)2, where F is the force, h is the separation, A = 3.7 x 10~20 J is the Hamaker constant for water-air-water [23] and R is the radius of curvature of the tip. Both R and d were used as fitting parameters. This equation assumes that there are water films present on the tip and the substrate of total thickness d. Figure 4B includes the JKR theory [24] for the force versus deformation. The radius obtained in the precontact data was used in the JKR theory, and the Young's modulus, which changes the slope, and the surface energy, which shifts the curve vertically, were used as fitting parameters. The influence of the capillary condensate, which would mainly shift the curve vertically, was not accounted for in the JKR theory (cf. Ref. [25]).