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

Area of Science:

• Physics
• Physical Chemistry
• Materials Science

Background:

• Understanding intersurface forces is crucial for nanoscale phenomena.
• Capillary bridges play a role in adhesion, friction, and self-assembly.
• Previous methods often involve geometric assumptions or experimental limitations.

Purpose of the Study:

• To determine forces between substrates mediated by a nanoscale water bridge.
• To investigate the influence of substrate separation on intersurface forces.
• To analyze the role of hydrogen bonding in force generation at small gaps.

Main Methods:

• Molecular dynamics simulations were employed.
• A nanoscale capillary water bridge was simulated between two planar, moderately hydrophilic substrates.
• The force between substrates was calculated as a function of their separation.

Main Results:

• Force changes continuously with separation, becoming discontinuous at small gaps due to unstable bridge configurations.
• An underlying oscillatory force with an increasing repulsive component was observed at separations of a few water molecules.
• Reduced hydrogen bond formation (HBN) near surfaces at small gaps contributes to the repulsive force.

Conclusions:

• Molecular dynamics simulations accurately predict forces in nanoscale water bridges.
• The observed force-discontinuity and oscillatory behavior are linked to molecular-level interactions and hydrogen bonding.
• Macroscopic force balance models are effective except for water layers.