  In swelling porous media, the potential for flow is much more than pressure,
and derivations for flow equations have yielded a variety of equations. In this
paper we show that the macroscopic flow potentials are the electro-chemical
potentials of the components of the fluid and that other forms of flow
equations, such as those derived through mixture theory or homogenization, are
a result of particular forms of the chemical potentials of the species. It is
also shown that depending upon whether one is considering the pressure of a
liquid in a reservoir in electro-chemical equilibrium with the swelling porous
media, or the pressure of the vicinal liquid within the swelling porous media,
a critical pressure gradient threshold exists or does not.
