Force exerted by a nanoscale capillary water bridge between two planar substrates

Abstract

Molecular dynamics simulation of a nanoscale capillary water bridge between two planar substrates is used to determine the resulting force between the substrates without arbitrariness regarding geometry and location of the free surface of the bridge. The substrates are moderately hydrophilic. The force changes continuously as the separation between the substrates changes except for small gaps where it becomes discontinuous because the bridge is unable to adopt stable configurations at any distance apart. Further exploration of the bridge and the force as the substrates approach each other reveals an underlying oscillatory force with an increasing repulsive component at separation distances equivalent to few water molecules. According to the average number of hydrogen bonds per water molecule (HBN), at very small gap sizes, water molecules which are very close to the surfaces are unable to maximize HBN thus contributing to the repulsive force. Our simulation results of force versus gap size agree with calculations based on other methods, some very different, and also reproduce the typical magnitude of the experimental force. Finally, a macroscopic force balance correctly describes the force-distance curve except for bridges constituted of water layers only.

What Governs Friction of Silicon Oxide in Humid Environment: Contact Area between Solids, Water Meniscus around the Contact, or Water Layer Structure?

“…Based on the ATR-IR spectroscopy study, Asay et al reported that the full coverage of monolayer adsorption occurs at RH < 10% . The silicon oxide surface is fully covered with the adsorbed water molecules, even at the lowest RH condition measured in this study (10%).…”

“…The lower wavernumber component can be attributed to the strongly hydrogen-bonded solidlike network and the higher wavenumber component is characteristic of the weakly hydrogen-bonded liquidlike structure. − The fact that the spectrum at 30% RH is dominated by the lower wavenumber component means that the thin water layer adsorbed on SiO x surface assumes the solidlike structure with stronger hydrogen bond interactions under this condition. As RH increases above 50%, the adsorbed water layer becomes thicker and the liquidlike structure becomes dominant. , …”

“…Based on the ATR-IR spectroscopy study, Asay et al reported that the full coverage of monolayer adsorption occurs at RH < 10%. 20 The silicon oxide surface is fully covered with the adsorbed water molecules, even at the lowest RH condition measured in this study (10%). Thus, the variance in τ s is expected to be small, at least not a strong function of RH as long as the solid surface is fully covered with the adsorbed water.…”

“…As RH increases above 50%, the adsorbed water layer becomes thicker and the liquidlike structure becomes dominant. 19,20 From Figures 6d and 7, it can be seen that ρ is large on the strongly hydrogen-bonded water layer at RH < 50% and becomes smaller as the liquid-like water structure grows at RH ≥ 50%. This could mean that the water meniscus is more difficult to drag on the solidlike structured water layer.…”

“…The adsorbed water layer on the clean silicon oxide surface assumes a solidlike structure at low RHs and a liquidlike structure at high RHs . Accordingly, this structural evolution of the adsorbed water layer strongly affects the adhesion force. , It has been suggested that a solidlike water bridge also could play an important role in friction behavior; however, a quantitative model of the convoluted effects of water layer structuring and capillary condensation on friction is not fully developed yet.…”