Capillary forces on elastic solids measured in molecular dynamics

The distribution of capillary forces that a liquid drop exerts on a solid substrate is still debated. While the force normal to the interface can be derived from a global argument, this is not the case for the tangential force component. Experiments in which the force is derived from the elastic deformation of the solid are difficult to perform and interpret, and have lead to different conclusions. To resolve this issue, we directly measure the force in Molecular Dynamics simulations of Lennard-Jones droplets in contact with a solid at varying contact angles. We find that the tangential force component is always pointed towards of interior of the drop, and can qualitatively be explained by density functional theory with the sharp kink approximation. This contradicts the classical view the that the capillary force on the solid acts parallel to the liquid interface.