Roughness Effect on the Pull-Off Force between Two Surfaces with Water Adsorbed between Them. A Molecular Dynamics Simulation Perspective

Abstract

Surface roughness has a high impact on the adhesion between particles and substrates. In the interaction of two surfaces in humid air, roughness usually controls the strength of the adhesion. In the low humidity range, where capillary theory is not enough to describe the phenomenon, it is necessary to advance in our understanding of how water affects the adhesion between two particles. The aim of this research is to give a molecular view of the role of roughness in the pull-off force between two substrates with water adsorbed between them. The method is molecular dynamics simulation of the pull-off. Roughness is described as an (almost) circular cavity or depression of depth from 0.00 to 0.95 nm and diameter of 7 nm. It was found that water molecules tend to be organized near the border of the depression. As a result, during pull-off simulation, monolayers and bilayers of water make close contact of the substrates impossible, preventing the occurrence of van der Waals forces. The layers of adsorbed water dominate pull-off with a solvation force. The results highlight the effect of the border of the cavity on the behavior of water molecules in nanocontact; this may be useful for the interpretation of experimental results and the development of pull-off models covering the full range of humidity.

Nanoscale-roughness influence on pull-off adhesion force in liquid and air

“…In fact, surface roughness influences strongly the capillary meniscus and the associated force so that even very slight roughness variations from the ideal surface (as little as ∼ 1nm rms roughness that can lead to peak heights up to ∼ 5 nm) can considerably diminish the strength of adhesion force [11]. This is because the surface asperity on a rough substrate will have a variable adhesive contact area [12]. Since the size of liquid menisci is limited by the asperities, the formation of the liquid bridge between two surfaces would change [13][14][15][16].…”