Static and Dynamic Wetting Behavior of Drops on Impregnated Structured Walls by Molecular Dynamics Simulation

Nanoscale drops on structured walls with which they interact through dispersive forces are investigated by molecular dynamics simulation. The drops are in the impregnation wetting regime, and the influence of the structures on the static and the dynamic wetting properties is studied. Drops of different size are simulated on six different walls which vary in the structure size and the wall morphology. Due to the nanoscale structure, there is a deviation of the interfacial tensions between the fluid and the wall from the value which would be observed on a macroscopic length scale. This phenomenon leads to contact angles which are greater than those predicted by the Cassie model. Accounting for the deviation of the interfacial tensions, the Cassie model is confirmed for drops on homogeneously structured walls, but it is shown to be inadequate for inhomogeneously structured walls. The dynamics of drop spreading is analyzed. It is observed that the presence of the wall structure poses hurdles to the spreading dynamics. The speed of spreading slows down with increasing structure size. The mechanism of drop spreading is observed to happen via the formation of local liquid protrusions of the drop and subsequent broadening of the protrusions. In the stable state, drop configurations are found which break the symmetry imposed by the wall geometry