Molecular dynamics and phase field simulations of droplets on surfaces with wettability gradient

To understand how the motion of a droplet on a surface can be controlled by wettability gradients is interesting in a variety of technical applications. Phase field (PF) models can be used to study such scenarios but their application requires suitable models of the properties of the interacting phases: vapor, liquid, and solid. In this work, the PF simulations are linked to molecular models by using an equation of state as well as a correlation for the viscosity, that are both consistent with results determined by molecular dynamics (MD) simulations. The motion of a nanoscale droplet on a surface with a wettability gradient is studied both by MD simulations and PF simulations and the results are compared. In both methods, the wettability gradient is solely determined by the surface tensions of the liquid–vapor, solid–liquid, and solid–vapor interfaces. Simulations are conducted for two different profiles of the wettability and at two different temperatures. The qualitative and the quantitative behavior such as the shape of the droplet and the velocity of the motion are in good agreement. This validates the PF model for the determination of nanoscale phenomena, and enables an efficient investigation of nanoscale as well as larger scenarios.