Super spreading of oil by condensed water drops

We studied the spreading of a thin film of hexadecane oil on a hydrophobic polystyrene surface, which is partially covered with tiny water droplets. In spite of the fact that the spreading coefficient of hexadecane is slightly negative on water, the oil actually spreads faster on the droplet covered polystyrene than on the pure substrate. The observed effect is partly due to the geometric factors that lead to greater reduction of oil-substrate interfacial energy in comparison to the increase of oil-vapor surface energy. However, it is more important to consider the reduction of the resistance to fluid motion in the presence of condensed droplets. We provide an explanation of the phenomenon by drawing an analogy to the spreading on a rough surface, in which each water drop nucleates and reinitiates the spreading process. The phenomenon is therefore an approximate 2D version of liquid invasion through porous media.