Hydration force and dynamic squeeze-out of hydration water under subnanometer confinement

We have performed molecular dynamics simulations to study the normal forces between two mica surfaces in water and the consequent dynamic squeeze-out process. A liquid-vapor molecular ensemble has been designed for this purpose. We find, however, that only the two-layer hydration water can support a repulsive hydration force, and beyond this thickness the overall forces are attractive. From the hydrodynamic pressure developed during a normal approach and dynamic squeeze-out, we estimate that the viscosity of confined hydration water in the thickness of 0.7-1.35 nm is about 5-16 times the bulk value. A steered diffusion model has been proposed, which directly predicts the critical velocity below which hydrodynamic effects can be eliminated in hydration force measurements