Surface Physics

Scientific interest in surface physics began very early with the XVIII century efforts trying to explain surface tension and capillarity. Molecules on the surface of a liquid are pulled inwards by the remaining molecules in the liquid phase and this produces a state of tension on the liquid surface. On the other hand, when interacting with a solid surface liquid molecules are subjected to attractive forces from the solid molecules, and, when these forces prevail over the forces between liquid molecules, this will give rise to a contact angle smaller than 90 \(^\circ \) and to capillary ascension inside small bore tubes. Surface tension forces liquid surfaces to contract to the minimal area and Laplace’s law relates the surface tension with the internal pressure that is produced and to the curvature of the liquid drops. Nevertheless, surface tension has a molecular origin and is independent of surface curvature. In this chapter, we try to give a deeper insight into the molecular scale for explaining surface tension under the light of van der Waals theory considering the surface as a region where the fluid density has a strong decay. It is then possible to relate the underlying molecular parameters with the Helmholtz energy in excess with respect to the liquid and vapor bulk phases. In liquid–solid and vapor–solid interaction, interfacial excess energies in the triple line region are also the controlling macroscopic mechanisms for the contact angle when a fluid interacts with a hydrophilic or hydrophobic surface and are important macroscopic parameters in dynamical problems where the contact angle gets away from its equilibrium value. A section is dedicated to exposing Cahn’s theory for finding the liquid–solid and vapor–solid interfacial energies. The chapter ends with a brief overview of emulsions.