Molecular dynamics simulations of the contact angle between water droplets and graphite surfaces

Wetting is a widespread phenomenon, most prominent in a number of cases, both in nature and technology. Droplets of pure water with initial radius ranging from 20 to 80 إ spreading on graphitic surfaces are studied by molecular dynamics simulations. The equilibrium contact angle is determined and the transition to the macroscopic limit is discussed using Young equation in its modified form. While the largest droplets are almost perfectly spherical, the profiles of the smallest ones are no more properly described by a circle. For the sake of accuracy, we employ a more general fitting procedure based on local averages. Furthermore, our results reveal that there is a possible transition to the macroscopic limit. The modified Young equation is particularly precise for characteristic lengths (radii and contact-line curvatures) around 40 إ.