Surface Forces in Liquids and Solids

The author's recent treatment of the forces in the surfaces of liquids is extended to the surfaces of solids and to interfaces between solids and liquids. Solid surfaces formed by cleavage at temperatures such that no appreciable migration of molecules takes place, will usually be under a state of uniform stress, not necessarily tensile and not numerically equal to the free energy of the surface. If appreciable atomic migration takes place, the surface of a crystalline solid melts, and the solid is covered with a thin liquid film under a tension force greater than that of the corresponding supercooled liquid, and such that the chemical potential of the molecules in the liquid film is equal to that of molecules in the crystal. This tension force is numerically equal to the free energy of the surface. If such a solid is subsequently cooled to a temperature at which atomic migration effectively ceases, it will have frozen in its surface a tension force corresponding to thermal equilibrium at some higher temperature. The Laplace Q force is discussed in terms of modern theories of atomic bonding and applied to the case of equilibrium at angle of contact between solid and liquid.