ABSTRACT
The wetting phenomenon is an important issue in various technological processes. In some fields, liquids are desired to spread over solid surfaces, e.g., lubrication oils on metallic surfaces or paint on paper. On the other hand, it is necessary for hydrophobic coatings to repel water such as Teflon film on frying pans. The behavior of bubbles on solid surfaces immersed in liquid often has important effects on the performance of industrial apparatus dealing boiling or condensation. In these problems regarding wetting, it is known that the behavior of a drop or bubble on a solid surface is dependent on the three interfacial tensions between solid, gas, and liquid phases, as shown in Fig. 1. The tangential force balance between these interfacial tensions on the three-phase contact line leads to the following well-known Young’s equation [1]:
σSV−σSL=σLV cos αY. (1)
σSV, σSL, and σLV indicate solid-vapor, solid-liquid, and liquid-vapor interfacial tensions, respectively. The environmental atmosphere is assumed to be filled with saturated vapor of liquid. When a drop is exposed to air, however, σLV usually does not change because a thin layer of saturated vapor may be formed around the drop [2]. In the right-hand side of Eq. (1), αY is the angle between the solid surface and the liquid-vapor interface measured from the inside of the liquid phase and is called the contact angle. When the difference between the two interfacial tensions on the left-hand side of Eq. (1) is large enough to make αY on the right-hand side small, the solid is favorably wetted by the liquid. As the drop size becomes sufficiently small and the curvature of the solid-gas-liquid contact line becomes quite large, we should add a term representing the effect of line tension to the above equation [3]. In this
