Wetting Invasion and Retreat across a Corner Boundary

The wetting phenomenon in the vicinity of a corner boundary is ubiquitous. The daily-life examples include the meniscus of water near the mouth of a container and the halt of the movement of a sliding droplet by the edge. In this study the wetting behavior near the edge is investigated both theoretically and experimentally by considering the volume growth of a droplet atop a conical frustum and the gradual immersion of a wedge. On the basis of free energy minimization, three different regimes are identified. When the contact line is away from the edge, Young's equation is followed. Once the contact line reaches the edge, the contact line is pinning at the edge due to the boundary minimum of the free energy. Consequently, the apparent contact angle exceeds the intrinsic contact angle and grows with increasing droplet volume or water level. As the apparent contact angle reaches the critical angle, which depends on the solid edge angle, liquid extends over the edge and the contact line advances along the new surface at its intrinsic contact angle. Similar behavior can be observed for wetting retreat but in a reverse order. The theoretical prediction has been experimentally confirmed.