Motion of nanodroplets near chemical heterogeneities.

We investigate the dynamics of nanoscale droplets in the vicinity of chemical steps which separate parts of a substrate with different wettabilities. Due to long-ranged dispersion forces, nanodroplets positioned on one side of the step perceive the different character of the other side even at a finite distance from the step, leading to a dynamic response. The direction of the ensuing motion of such droplets depends not only on the difference between the equilibrium contact angles on these two parts but in particular on the difference between the corresponding Hamaker constants. Therefore, the motion is not necessarily directed toward the more wettable side and can also be different from that of droplets which span the step.