Vanishing of Capillary Hysteresis in Porous Media
Tetsu K. Tokunaga, Keith R. Olson, and Jiamin Wan
Contact: Tetsu K. Tokunaga, 510-486-7176, TKTokunaga@lbl.gov
Research Objectives
The degree of water saturation (S) exerts strong influences on water flow, transport of solutes and heat, and mechanical properties of porous media. The monotonic decrease of the matric potential (y) with decreased saturation reflects the combination of capillary and adsorptive influences in lowering the free energy of soil water. In the early decades of soil physics, it was assumed that the S(y) relation in a given system represented a series of unique equilibrium conditions. Since the classic 1930 paper of Haines, hysteresis in S(y) has been regarded as a basic aspect of interactions between water and variably saturated porous media. At any given potential, the equilibrium saturation level obtained by draining a system is greater than or equal to that obtained by wetting an initially dry system to the same potential. In an attempt to gain a better understanding of the hysteresis phenomenon, we consider conditions that lead to its disappearance.
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 The Haines number is comprised of fluid-fluid density differences and surface tension, acceleration, and grain-size. Grain-size dependence of the Haines number for (a) standard conditions (air-wave, ordinary gravity), (b) low surface tension (surfactant solution, ordinary gravity), (c) much higher body force (air-water, 1,000g centrifuge), and (d) an oil-water system conditions (ordinary gravity). The horizontal line at Ha=16 separates systems that do and that do not exhibit hysteresis.
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