Effect of the initial conditions on the onset of motion in sessile drops on tilted plates

The shape of a drop on an inclined hysteretic surface has been studied both theoretically and experimentally in this paper. Surface Evolver (SE) has been used to model the shape of the drop. The triple line was initially circular. The energy minimization method from SE is coupled with a triple line dynamics model to incorporate contact angle hysteresis (CAH) into the simulations. Experiments have also been performed with sessile drops on tilting surfaces to validate the results obtained from the SE model. From this study, two critical inclination angles are identified that describe the incipient motion of the drop. The moving angle is the first critical inclination angle at which the triple line is on the verge of being deformed from a given initial shape. The sliding angle is the second critical inclination angle at which the entire drop is in a state of impending motion. It was observed that the moving angle is a strong function of the initial contact angle. It was observed to increase initially and then decrease as the initial contact angle is increased. The sliding angle decreased monotonically as the initial contact angle is increased. A quantitative correlation is developed to explain the sliding angle as a function of the initial conditions. The predictions of this correlation have been shown to compare well with the experimental data from this study as well as with the literature.