Free Energy Barriers in Wetting-Mode Transitions of Droplets On Grooved Surfaces

Wetting behavior of a liquid droplet on a rough or physically-patterned surface is typically characterized by either the Cassie mode, in which the droplet resides on top of the surface features, or the Wenzel mode, in which the droplet penetrates into the surface features. For fixed surface topology and droplet size, only one of these modes represents the global free-energy minimum. The other state can be metastable and sufficiently long-lived to be observed experimentally, due to a free-energy barrier that hinders the transition to the global free-energy minimum. In this work, we examine the wetting behavior of a droplet on a grooved surface using both a nano-scale molecular dynamics (MD) approach and a macro-scale model based on free energy minimization. We observe the metastable states in our molecular dynamics (MD) simulations, and use forward flux sampling to study the kinetics of the Cassie-Wenzel transition. We find that the global-minimum wetting states emerging from our MD simulations are consistent with those predicted by our macroscopic model.  We also find that the free-energy barrier for transition to the global free-energy minimum depends on both droplet size and surface topology, with a transition-state ensemble consisting of droplets that are on the verge of initiating/breaking contact with the substrate at the bottom of the grooves.