Nanodroplet impact and sliding on structured polymer surfaces

A comprehensive understanding of the wetting dynamics of rough hydrophobic surfaces is necessary for the optimization of practical applications. Molecular dynamics simulations have been used to study the impact and sliding of a nanosized water droplet on nanostructured polyethylene (PE) and poly(vinyl chloride) (PVC) polymer surfaces. The impact velocity on collision with pillar surfaces mainly influences the maximal deformation of the drop from spherical and its intrusion depth between the pillars. There is no effect on the observed equilibrium state, with the exception of the most hydrophobic PE surface, but the equilibrium shape of a droplet at the Wenzel state may be asymmetrical. A bounce of the water droplet from the most hydrophobic PE surface with smaller pillars is observed at intermediate velocity. A bounce occurs at a lower impact velocity if the collision angle is decreased or the mass of the droplet is increased. The friction against sliding was higher on the PVC surfaces than on the PE surfaces due to the less hydrophobic nature of the corresponding polymer and clear pinning of the receding contact line. The sliding distances on the PVC pillar and grooved surfaces were all short and within a small range, whereas on the surfaces the differences were more substantial. The highest friction was observed on the grooved surface for sliding orthogonal to the grooves and the lowest friction was observed on the pillar surface with the smallest pillars. The friction increased with the size of the pillars due to the decreased hydrophobicity and increased resisting force of the pillar edges. The collisions with the pillar edges decreased the sliding distance on the more hydrophobic pillar surface such that it was equal to that on the grooved surface in the groove direction.