A molecular dynamics study of the motion of a nanodroplet of pure liquid on a wetting gradient.

The dynamic behavior of a nanodroplet of a pure liquid on a wetting gradient was studied using molecular dynamics simulation. The spontaneous motion of the droplet is induced by a force imbalance at the contact line. We considered a Lennard-Jones system as well as water on a self-assembled monolayer (SAM). The motion of the droplet for the Lennard-Jones case was found to be steady with a simple power law describing its center-of-mass position with time. The behavior of the water droplet was found to depend on the uniformity of the wetting gradient, which was composed of methyl- and hydroxyl-terminated alkanethiol chains on Au(111). When the gradient was nonuniform the droplet was found to become pinned at an intermediate position. However, a uniform gradient with the same overall strength was found to drive a droplet consisting of 2000 water molecules a distance of 25 nm or nearly ten times its initial base radius in tens of nanoseconds. A similar result was obtained for a droplet that was twice as large. Despite the many differences between the Lennard-Jones and water-SAM systems, the two show a similar overall behavior for the motion. Fair agreement was seen between the simulation results for the water droplet speed and the theoretical predictions. When the driving force was corrected for contact angle hysteresis, the agreement was seen to improve.