A new view on the surface stresses in condensed media

Abstract

The present review deals with the inadequacy of the conventional notions of the deformation of the boundary layer of a condensed medium due to “uncompensated” surface forces. It is pointed out that, in reality, both the boundary and inner molecules of the medium are in their intrinsic equilibrium positions, which should be treated as deformation and internal stress reference points. In the absence of external forces a body is generally free from deformations and internal stresses, including a boundary layer. It is groundless to regard the surface layers to be deformed and stressed relative to the inner parts of a body, or, vice versa, the inner parts relative to the surface layers, only on the basis of the difference of equilibrium intermolecular distances in these two cases. Hence, the very idea of “uncompensative” surface stresses loses its sense.

The present-day theoretical descriptions of surface stresses are shown to be unfounded. Proceeding from the virial theorem and equilibrium conditions in the system, it is shown that in the absence of external forces, equilibrium pressure inside and near the boundary of a condensed medium is equal to zero. The attempts to connect thermodynamical and mechanical definitions of surface tension are critically evaluated. Thermodynamics of small particles is briefly discussed, considering that they are free from internal stresses.

Key Words:

• Surface pressure
• surface tension
• phase transitions
• small particles