Onset of sliding motion in sessile drops with initially non-circular contact lines

Abstract

The onset of motion of a drop with an initially non-circular three phase contact line was studied experimentally and numerically. Two drops of volume 10 μl were made to coalesce and form a composite 20 μl drop. The contact line of this drop was approximately elliptical and the local contact angle along the contact line was not a constant (as would have been the case with a circular contact line). The orientation of the drop to the impending direction of motion was varied. Inclined plate experiments were performed and the moving and sliding angles were noted in each case. It was observed that the moving and sliding angles of the drop were strongly dependent on this orientation. Specifically, the local conditions on the contact line at the front and back edges of the drop as well as the drop profile width were found to be the determining parameters. Surface evolver simulations were performed to understand the results of the experiments. It was found that the evolution of the contact line for non-circular drops was rather counter-intuitive when compared to the results from a drop with a circular contact line and resulted from a competition between gravity and the local contact angle hysteresis forces.