The movement of water in swelling materials

A theory is developed to describe the one-dimensional movement of electrolyte in clay-electrolyte systems that are free to swell in the direction of electrolyte movement. This theory develops from the observation that Darcy's law describes flow of fluid relative to the comparatively immobile particles of clay, and is based on a scale of length defined with reference to the distribution of mass of the clay rather than the more conventional fixed scale of length. It is shown, theoretically and experimentally, that in this scale of length the appropriate equation is the diffusion equation, for the use of which there exists a large body of information in soil physics and other literature.