Surface and Interface Stresses

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Perhaps the most fundamental difference (from a thermodynamic point of view) between a solid surface and a liquid surface involves the distinction between the surface free energy and the surface stress. The reversible work per unit area involved in forming a surface, which exposes new atoms, is the surface free energy y. This parameter describes the reversible work to form a new solid surface by a process such as cleavage. The surface stress I is the reversible work per unit area required to elastically stretch a surface. When a fluid surface is stretched, new atoms or molecules arrive at the surface such that the number of atoms per unit area remains constant. As a result, it can be considered that for a fluid the surface free energy is the same as the surface stress. On the other hand, when a solid surface is elastically stretched, the actual number of atoms per unit area is altered and, in general, I =f. y. When a solid of finite size is elastically deformed, work is performed against both volume and surface forces. Under most conditions the volume term will strongly dominate over the surface term. However, for solids of small enough extent, the surface term can become important and induce a bulk stress of order I/t that elastically changes the eqUilibrium lattice spacing. Here t corresponds to the characteristic length