“…The first attempt to solve the problem of adhesive interaction between two elastic spheres was undertaken by Derjaguin [24], who has formulated the following principle: The vertical work done by the external load is equal to the sum of the virtual change of the potential elastic energy and the virtual work that will be consumed by the increase of the surface attractions. Since then, this variational principle has become the cornerstone of many studies of adhesive contact [25,26]. Let U T denote the total energy, which is obtained by summation of the stored elastic energy, U E , the mechanical energy of the applied load, U M , and the surface energy, U S , so that U T = U E + U M + U S .…”