1 Non-Equilibrium Thermodynamics and the Mesoscopic Level of Description

In 1931, Onsager published two papers on simultaneous irreversible processes ^[1] in which he formulated the well-known reciprocal relations between phenomenologi-cal (Onsager) coefficients establishing in this way the basis for the formulation of a thermodynamical theory of irreversible processes, referred nowadays to as non-equilibrium thermodynamics. ^[2] The success of the theory is mainly due to the simplicity of its method, which runs parallel to that of equilibrium thermodynamics, to the many situations pertaining to the macroscopic world to which it applies, and to its solid connection with statistical mechanics and kinetic theory. Irreversible processes as for example heat conduction, mass diffusion and electrical phenomena, which constitute fundamental mechanisms of transport in bulk phases and at interfaces ^[3] and are the basis of many industrial processes, can convincingly be explained under the framework of the theory. Non-equilibrium thermodynamics nowadays constitutes a powerful tool with many applications in physical, physicochemical and biological systems as well as in engineering processing.