Surface Energy and Surface Stress

This chapter provides thermodynamic definition of surface energy. The Gibbs free energy quantifies the maximum useful work that may be extracted from a system while the Helmholtz free energy dictates the total work involved in perturbing the system subject to the condition that the temperature starts and ends at the temperature of its surroundings. The specific surface energy changes by different amounts when the surface is strained in different senses, so the corresponding surface stress components vary likewise. In this respect, the stress at a solid surface differs qualitatively from that of a liquid surface, where the second term on the right-hand side of the Shuttleworth equation necessarily vanishes. The chapter considers a few case studies in which the surface stress of chiral systems has been calculated from first principles. For an unstrained surface having no mirror or glide planes the surface stress is entirely unconstrained by surface symmetry.