Capillary Adhesion at the Nanometer Scale

Molecular dynamics simulations are used to study the capillary adhesion from a nonvolatile liquid meniscus between a spherical tip and a flat substrate. The atomic structure of the tip, the tip radius, the contact angles of the liquid on the two surfaces, and the volume of the liquid bridge are varied. The capillary force between the tip and substrate is calculated as a function of their separation h. The force agrees with continuum predictions for h down to ~ 5 to 10nm. At smaller h, the force tends to be less attractive than predicted and has strong oscillations. This oscillatory component of the capillary force is completely missed in the continuum theory, which only includes contributions from the surface tension around the circumference of the meniscus and the pressure difference over the cross section of the meniscus. The oscillation is found to be due to molecular layering of the liquid confined in the narrow gap between the tip and substrate. This effect is most pronounced for large tip radii and/or smooth surfaces. The other two components considered by the continuum theory are also identified. The surface tension term, as well as the meniscus shape, is accurately described by the continuum prediction for h down to ~ 1nm, but the capillary pressure term is always more positive than the corresponding continuum result. This shift in the capillary pressure reduces the average adhesion by a factor as large as 2 from its continuum value and is found to be due to an anisotropy in the pressure tensor. The cross-sectional component is consistent with the capillary pressure predicted by the continuum theory (i.e., the Young-Laplace equation), but the normal pressure that determines the capillary force is always more positive than the continuum counterpart.