Direction Dependence of Adhesion Force for Droplets on Rough Substrates.

Determination of solid surface free energy is still an open problem. At present, there are two leading theories on how to determine the adhesion of droplets on rough substrates: one theory stresses that the droplet adhesion force lies in the areas of contact and interaction energy between liquid and solid molecules, whereas the other holds that the length of the edge of drops is essential. In this work, we unify the two theories through lattice Boltzmann simulations and demonstrate that the adhesion force could depend on either the contact area or the contact line, depending on the direction of the adhesion force measured, that is, by vertically separating the two materials or laterally sliding the droplet on the substrate. We reveal that for separating droplets away from rough substrates, the vertical adhesion (pull-off) force depends more significantly on the contact area rather than on the contact line. However, for sliding a droplet on substrates, the lateral adhesion force depends on the contact line while being independent of the contact area. [ABSTRACT FROM AUTHOR]