Читать книгу Non-equilibrium Thermodynamics of Heterogeneous Systems - Signe Kjelstrup - Страница 17

This chapter gives thermodynamic relations for heterogeneous systems that are in global equilibrium, and discusses the meaning of local equilibrium in homogeneous phases, at surfaces and along three-phase contact lines.

The heterogeneous systems that are treated in this book exchange heat, mass and charge with their surroundings. The systems have homogeneous phases separated by an interfacial region. They are electroneutral, but polarizable. The words surface and interface will be used interchangeably to indicate the interfacial region. Gibbs [65] calls the interfacial region “the surface of discontinuity”. Thermodynamic relations for homogeneous phases, as well as for surfaces, are mandatory for the chapters to follow. Such equations are therefore presented here [65, 96]. Equations for the threephase contact line are also given, but are less central. The equations are given first for systems that are in global equilibrium, and next for nonequilibrium systems where only local equilibrium applies.

A thermodynamic description of equilibrium surfaces in terms of excess densities was constructed by Gibbs [65]. This description treats the surface as an autonomous thermodynamic system. We use this description in terms of excess densities as our basis also in non-equilibrium systems. This implies for instance that the surface has its own temperature. All excess densities of a surface depend on this temperature alone and not on the temperatures in the adjacent phases.¹ We present evidence for this assumption using non-equilibrium molecular dynamics simulations (Chapter 22) and the square gradient model of van der Waals (Chapter 23).

We extend Gibbs’ formulation for the surface to the contact line. Equilibrium relations for the three-phase contact line can be formulated for excess line densities. The line, described in this manner, is also an autonomous thermodynamic system.