Wetting on Flat and Smooth Surfaces

Abstract

The Young's contact angle gives a notion of wettability when a liquid wets and spreads on a solid surface. However, it is also a source of arguments and controversies in the literature. The objective of this chapter is to clarify a couple of fundamental issues that have been debated for more than a century. These issues are: (1) is the liquid droplet in thermodynamic or mechanical equilibrium in the static position as described by Young? And (2) what determines the contact angle, the contact line or the contact area? Studies of the dynamics of the wetting process, by molecular kinetic theory, hydrodynamic theory and experiments, reveal that liquid usually spreads after it wets the surface. The action of spreading is retarded by friction due to molecular adhesion and/or roughness at the liquid--solid interface. The liquid droplet will cease to advance and end up in the final static position with a static contact angle $θ$ when all of its kinetic energy is dissipated. Evidence is provided that the liquid droplet is metastable in this static position. When sufficient vibration energy is provided to the liquid droplet, the contact line is shown to de-pin and the droplet is driven to its thermodynamically stable state, resulting in the most stable droplet with an equilibrium contact angle $θ$eq. Two simple and clever experimental designs from the literature are reviewed. Both experiments involve the fabrication of chemically heterogeneous surfaces comprising millimeter-size spot of material-1 on a surface of material-2. The contact angles for the spot and the bulk surface are very different. Advancing contact angle measurement was performed from the center of the spot. As the contact line is advancing, the contact angle switches from material-1 to material-2 as soon as the advancing contact line reaches the bulk surface. This is despite the fact that the change in surface energy for the contact area is very small. The results clearly demonstrate that contact angle is determined by the energetics around the contact line, not the contact area underneath the liquid droplet. Recent thermodynamic modelling also confirms that contact angle is a one-dimensional phenomenon.