A new view of the “surface energy-surface tension” problem

Abstract

4951 The notions of the "surface energy" (SE) and "sur face tension" (ST) were discussed well before seminal Gibbs works [1] who defined SE σ only for liquids: "…when one of the masses is solid there is no such equivalence between the stretching of the surface and the forming of new surface." He emphasized: "… tensions of the surfaces may evidently have different values in different directions, and are entirely different from the quantity which we denote by σ, which is not connected with any idea of direction" (see for details [2,3,4]). Shuttleworth [5] was the first to introduce as the ST of solids and derived his equation defining the relation between σ and γ. It is the basic equation of the theory of the ST of solids accepted by the majority of experts, with a few exceptions, e.g. in [3,4] this equation was shown to contradict all available experimental data. In the last decade, some new approaches and equations for ST of solids have appeared, but they have not yet been generally appreciated. Besides, the Shuttleworth equation follows from them as a particular case, and this raises doubts because an incorrect equation can not be derived from a correct theory.Brown [6] discussed simple ideas that might sound now like new ones in application to SE and ST of liq uids and solids. For example: "… in accordance with the principle that every system moves towards a state of minimum potential energy the surface of a liquid shows a tendency to contract. At this stage, SE is therefore the reality and that ST is a fiction which is mathematically equivalent to SE… It is erroneous to conclude that ST is non existent because … postulate of molecular cohesive forces leads in the first place to an explanation of SE. On the contrary, if SE exists, it must to perform work any theory which accounts for SE in liquids must, of necessity, also account for ST. Free energy per unit area and tension per unit length in a liquid surface are physically equivalent (not merely mathematically)." Brown concluded: "My sole pur pose has been to ask those who concede a physical 1 The article is published in the original. existence to SE, but not ST, whether they have any more reason for doing this than for regarding, say, the weight of a body as a fiction, but its gravitational potential energy as a reality. I believe that ST and SE are no more or less real than any other types of force and energy". One of Brown's opponents insisted [6]: "…I doubt whether the word "real" has any place in physics, which I take to be the science of measurable properties. If by "real" you mean measurable, and by "fiction" not measurable, then you must admit that ST of liquids is real because it is directly measurable, while SE is much more doubtful."The latter point of view became more popular in the sixties and seventies, when Bikerman wrought sev eral articles expressed doubts in the existence of the SE, e.g. he stated [7]: "… no energy can be attributed to the surface of a solid, or at best such surface energy cannot be measured even if it existed." And later: "...