Solid/fluid interfacial energy

The technique of contact mechanics has also been applied to the direct mechanical determination of solid-fluid interfacial energies, and the results compare favorably with those obtained by contact angle measurements [19]. 

The balance of forces between surface tensions at the contact line results either in the Neumann triangle for a liquid/liquid/liquid or liquid/liquid/gas system or in the Young-Dupre equation on a liquid/liquid/solid or a liquid/gas/solid system (Fig. 1). While the Neumann triangle represents a true balance of forces, the Young-Dupre equation is little more than a definition of the (o As ctbs) term, a difference between the respective solid/fluid surface free energies and not truly solid/fluid interfacial tensions. 

Whilst certain of these methods involve measurements only at the liquid-vapour or liquid-liquid interfaces involved in the static methods we must usually consider besides the interfacial energy of the two fluids, that between each of them and a solid... 

The way in which a particular system behaves depends on the interfacial energies between the solid substrate and any contacting liquid, and between the liquid and the second fluid (air). By manipulating these factors, the wetting process can be controlled. This may be achieved by the use of surfactants. 

Alternatively, thermodynamic phase equilibrium in a model system can be evaluated by beginning the simulation with two (or more) phases in the same simulation volume, in direct physical contact (i.e., with a solid-fluid interface). This approach has succeeded [79], but its application can be problematic. Some of the issues have been reviewed by Frenkel and McTague [80]. Certainly the system must be large (recent studies [79,81,82] have employed from 1000 up to 65,000 particles) to permit the bulk nature of both phases to be represented. This is not as difficult for solid-liquid equilibrium as it is for vapor-liquid, because the solid and liquid densities are much more alike (it is a weaker first-order transition) and the interfacial free energy is smaller. However, the weakness of the transition also implies that a system out of equilibrium experiences a smaller driving force to the equilibrium condition. Consequently, equilibration of the system, particularly at the interface, may be slow. 

In practice the fluid adhesives are spread over the substrates and constrained in some way so that the problem is to determine whether the surface free energies and interfacial energy of the phases in question will lead to complete wetting. A fluid adhesive spread over a real (nonplanar) substrate is shown schematically in Figure 1. The system is depicted in a state in which the solid is not completely wetted (upper) and the wetted state (lower). 

---