Adhesion between surfaces in undersaturated vapors—a reexamination of the influence of meniscus curvature and surface forces

Abstract

The pull-off force between molecularly smooth mica surfaces has been measured as a function of the relative vapor pressure of cyclohexane, n-hexane, and water. For the experiments with water both normal mica (with surface potassium ions) and ion-exchanged mica (covered with hydrogen ions) have been used. The results of an earlier study (L. R. Fisher and J. N. Israelachvili, Colloids Surf. 3, 303 (1981)) have been shown to lead to erroneous conclusions due to the use of a rolling and shearing spring. Here a nonrolling and virtually shear-free double cantilever spring has been used. At high relative vapor pressures ( p/ p s ≳ 0.7) the pull-off force for all vapors is dominated by the Laplace pressure in a capillary-condensed annulus. There is a small but noticeable contribution from a solid-solid interaction across the annulus, but surface deformations do not appear to affect the measured pull-off force. As the relative vapor pressure decreases from ∼0.7 to 0, the pull-off force varies smoothly and approaches its value in the “dry” state. There is no difference in behavior between water and the nonpolar liquids other than can be explained in terms of the solid-solid contribution to the adhesion. It is concluded that such measurements do not give any immediate information on the validity of the bulk surface tension for very high ( r < 4 nm) curvature systems, at least not in the absence of a complete treatment of the surface deformation problem.