On the cohesion of fluids and their adhesion to solids: Young's equation at the atomic scale.

Using large-scale molecular dynamics simulations, we model a 9.2nm liquid bridge between two solid plates having a regular hexagonal lattice and analyse the forces acting at the various interfaces for a range of liquid-solid interactions. Our objective is to study the mechanical equilibrium of the system, especially that at the three-phase contact line. We confirm previous MD studies that have shown that the internal pressure inside the liquid is given precisely by the Laplace contribution and that the solid exerts a global force at the contact line in agreement with Young's equation, validating it down to the nanometre scale, which we quantify. In addition, we confirm that the force exerted by the liquid on the solid has the expected normal component equal to γsinθ, where γ is the surface tension of the liquid and θ is the equilibrium contact angle measured on the scale of the meniscus. Recent thermodynamic arguments predict that the tangential force exerted by the liquid on the solid should be equal to the work of adhesion expressed as Wa=γ(1+cosθ). However, we find that this is true only when any layering of the liquid molecules close to liquid-solid interface is negligible. The force significantly exceeds this value when strong layering is present.