Advancing and receding wetting behavior of a droplet on a narrow rectangular plane

The contact angle (CA) measurements are generally performed on a large planar surface of a specific substrate with the width larger than the droplet size. In this study, the contact angle hysteresis on a narrow rectangular plane with a width smaller than the droplet size is experimentally studied through the inflation-deflation process by the needle-syringe method. The inflation process by stepwise addition of the liquid to the droplet leads to the contact line advancing outwardly along the major axis with advancing angle (theta(a)). Although the droplet width is constrained by the edge of the plane, the CA along the minor axis (theta(w)) increases and its value is greater than theta(a) (theta(w) > theta(a)). Deflation process by stepwise withdrawal of liquid from the droplet results in the contact line retracting inwardly along the major axis as the CA reduces to receding angle (theta(r)). In the meantime, the CA along the minor axis decreases as well. Both advancing and receding angles acquired from the narrow rectangular plane are confirmed with those obtained form the typical large surface of acrylic glass. On the basis of free energy minimization and liquid-induced defects model, Surface Evolver simulations are performed to reproduce the behavior of droplet on the narrow rectangular plane during the inflation-deflation process. The results of experiment and simulation agree with each other very well.