(452g) Investigation of Free-Energy Barriers for Droplet Wetting-Mode Transitions On Structured Surfaces Using Forward Flux Sampling

Liquid droplets exhibit multiple wetting modes on textured surfaces, depending on the surface topography, intrinsic contact angle, and droplet size.  In addition, it has been observed experimentally that the wetting state of a droplet can depend on its initial position on the substrate.  In this case, the wetting state with the lowest free energy may not be achieved due to the existence of a free-energy barrier between two wetting modes.  In this work, we use molecular dynamics simulations to study the wetting states of droplets on grooved hydrophobic solid surfaces.  We observe both Cassie and Wenzel wetting modes, in which the droplet resides on top of the grooves or penetrates within the grooves, respectively.  As in the experiments, the wetting state we observe can depend on the initial configuration of the droplet, indicating the presence of long-lived metastable states.  We use the forward flux sampling method to study transitions between the Cassie and Wenzel states.   Our studies show that metastable wetting states can exist over human time scales, and that the free-energy barrier for transitions between these states depends on topological parameters characterizing the grooved surface.