States of a Water Droplet on Nanostructured Surfaces
S Chen and JD Wang and DR Chen, JOURNAL OF PHYSICAL CHEMISTRY C, 118, 18529-18536 (2014).
Wetting behavior of a water droplet on nanostructured surfaces is studied by molecular dynamics simulations. The results show that the equilibrium states can be classified into four categories: the Wenzel, the metastable Cassie, the globally steady Cassie, and the Cross state. For the Wenzel state, the equilibrium state is independent of the initial state of the water droplet, and always in the Wenzel state no matter if the initial state is the Wenzel or the Cassie state. For the metastable Cassie state, the equilibrium state is dependent on the initial state: the equilibrium state presents as the Cassie state only when the initial state is the Cassie state, otherwise it presents as the Wenzel state. For the globally steady Cassie state, it is always in an equilibrium state of the Cassie state regardless of the initial state, which indicates that the transition from the Wenzel to the Cassie state can spontaneously happen. The results also show that the equilibrium state remains unchanged with the droplet size increasing for these three states. The Cross state is a state that can only be found on nanostructured surfaces, and the existence of this state should be attributed to the structured water molecules in the nanostructures. Furthermore, calculation from a criterion derived from a continuum mechanical analysis for the Cassie state is given, and the results demonstrate that the predictions from the criteria for the Cassie state agree with the results from molecular dynamics simulation for the globally steady Cassie state.
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