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.

Modeling experimental results of pull-off versus relative humidity obtained by atomic force microscopy (AFM) has suggested that the classic interaction force models (van der Waals and capillarity) do not capture the water behavior confined between two surfaces at a distance D ≈ 1 nm. In this paper, pull-off generated by bridges of water confined between two flat substrates is studied using molecular dynamics simulations, varying the wetting conditions and the number of water molecules of the bridge. The method used imitates the approach and retraction curves in AFM. The confined water exhibits an oscillatory solvation force for D < 1.3 nm for both hydrophilic and hydrophobic surfaces. For hydrophilic surfaces, the pull-off corresponds precisely to the maximum of attractive force produced at D ≈ 0.7 nm, where the bridge is composed of two water layers. These results are applicable to the tip−sample nanocontact of experimental systems for substrates at a contact angle >40°.

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Molecular dynamics simulations allow determination of the force between a tip and a sample mediated by a fluid or a nanoscale capillary bridge at any separation distance in approach−retract cycles. However, without further consideration, the procedure leads to force−distance curves without control of tip-to-sample distance and tip oscillations. Here we show that the solution of a macroscopic model describing the dynamics of the tip−sample interaction can precisely guide the choice of optimum parameters, for instance, tip elastic constant and approach speed, for controlled simulations. The method is applied to the interaction between two substrates in two cases of intervening fluid: a water bridge and a Lennard-Jones bridge. The method can be very useful in the determination of force−distance curves in more complex systems.