Mahesh V Panchagnula

In this paper, we study equilibrium three-dimensional shapes of drops on hysteretic surfaces. We develop a function coupled with the publicly available surface energy minimization code Surface Evolver to handle contact angle hysteresis. The function incorporates a model for the mobility of the triple line into Surface Evolver. The only inputs to the model are the advancing and receding contact angles of the surface. We demonstrate this model's versatility by studying three problems in which parts of the triple line advance while other parts either recede or remain stationary. The first problem focuses on the three-dimensional shape of a static pendant drop on a vertical surface. We predict the finite drop volume when impending sliding motion is observed. In the second problem, we examine the equilibrium shapes of coalescing sessile drops on hysteretic surfaces. Finally, we study coalescing puddles in which gravity plays a leading role in determining the equilibrium puddle shape along with hysteresis. © 2012 Springer-Verlag.

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.

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. © 2014 Elsevier B.V.

Wetting of smooth, chemically heterogeneous surfaces was studied experimentally and computationally during the advancing and receding processes. The motion of the triple line is known to play an important role in determining the macroscopic contact angle due to its ability to be pinned at various defect locations on real surfaces. This effect is known to cause contact angle hysteresis. The shape of the triple line during these pinning/de-pinning events on various chemically heterogeneous surfaces was captured using an experimental and a computational technique. The experimental study employed a Modified Wilhelmy Plate Technique. The novelty in the current experimental setup lies in its ability to capture the microscopic triple line shape and its evolution in addition to measuring the local contact angles, which were both studied. The triple line shape was observed to be very sensitive to minor imperfections of the substrate. In addition, Surface Evolver was used to study the triple line shape computationally. Evolver was used to solve the complete three-dimensional problem by minimization of the total energy taking into consideration, both gravity and contact angle hysteresis. The studies showed that the temporal evolution of the triple line was significantly different during the advancing and receding processes based on the nature of the chemical heterogeneity. The results from the current work could be used in the design and fabrication of chemically heterogeneous surfaces for desired wetting applications. © 2014 Elsevier B.V.