Capillary Condensation and Snap-off in Nanoscale Contacts

When a surface is placed in a vapor, several layers of molecules may adsorb depending on the intermolecular forces involved. As two such surfaces are brought together, a critical point is reached at which the gas condenses between the surfaces, forming a capillary across the gap. A cohesive force is associated with the condensed bridge. The reverse process wherein the capillary bridge degenerates as the surfaces are moved apart is called snap-off. These processes play a profound role on scales from the nano to the macro. We have studied this phenomenon via isostrain grand canonical Monte Carlo statistical mechanical simulations for Lennard-Jones fluids. Specifically, we have examined capillary condensation and snap-off between nanocontacts, infinite rectilinear nanowires, and finite rectilinear nanoplatelets, where macroscale concepts and theories are just about impossible to apply. These results are compared to condensation between infinite parallel plates. We discuss our results in terms of the Kelvin equation and van der Waals film-thickening model.