Nanocapillary adhesion between parallel plates

TL;DR

This study uses molecular dynamics simulations to analyze nanometer-scale capillary adhesion between parallel plates, revealing deviations from macroscopic theory at very small separations due to molecular layering effects.

Contribution

It demonstrates the limitations of macroscopic theory at nanometer scales and highlights the importance of molecular layering and anisotropic pressure tensors in nanocapillary adhesion.

Findings

Macroscopic theory predicts meniscus shape accurately down to 1-2nm separations.

Total capillary force differs from theory below about 5nm due to molecular layering.

Pressure tensor becomes anisotropic at small separations, affecting capillary adhesion.

Abstract

Molecular dynamics simulations are used to study capillary adhesion from a nanometer scale liquid bridge between two parallel flat solid surfaces. The capillary force and the meniscus shape of the bridge are computed as the separation between the solid surfaces is varied. Macroscopic theory predicts the meniscus shape and the contribution of liquid/vapor interfacial tension to the capillary force quite accurately for separations as small as 2 or 3 molecular diameters (1-2nm). However the total capillary force differs in sign and magnitude from macroscopic theory for separations less than about 5nm (8-10 diameters) because of molecular layering that is not included in macroscopic theory. For these small separations, the pressure tensor in the fluid becomes anisotropic. The components in the plane of the surface vary smoothly and are consistent with theory based on the macroscopic surface tension. Capillary adhesion is affected by only the perpendicular component, which has strong oscillations as the molecular layering changes.