High Quality Content by WIKIPEDIA articles! Surface stress was first defined by Josiah Willard Gibbs as the amount of reversible work per unit area needed to elastically stretch a pre-existing surface. A similar term called “surface free energy”, which represents the excess free energy per unit area needed to create a new surface, is easily confused with “surface stress”. Although surface stress and surface free energy of liquid-gas or liquid-liquid interface are the same, they are very different in solid–gas or solid–solid interface, which will be discussed in details later. Since both terms represent a force per unit length, they have been referred to as “surface tension”, which contributes further to the confusion in the literature.