Water flow in unsaturated soils in microgravity environment

In this study, two types of soils with varying soil water potentials were used for evaluating the effect of gravity on water flow through unsaturated soils. Experiments were conducted in both 1- and 0-g environments. Water content distributions were evaluated as a function of distance from the source of water intake and time. The experimental results indicated that the capillary potential and the advective forces due to interfacial tension gradients are overshadowed by the gravitational potential in a 1-g environment. The fast water movement in the 0-g condition is attributed to the capillary potential as well as to the advective forces that developed. In addition, microstructural changes have contributed to water flow in the 0-g condition. Depending on soil type, the magnitude of such an effect (i.e., water movement) varies from three- to four-fold. To analyze the experimental results, a one-dimensional model, based on Darcy's law and the conservation of mass equations, was developed and solved numerically by the finite difference method. A nondimensional Bond number was extracted from the resulting flow equation and used as a basis for incorporating the gravitational component of the flow process into the formulation. The numerical results compare quite well in some instances with the experimental results. In other cases, significant departures are noted. The departure was attributed to the significant changes in microstructure of soil samples under the 0-g condition. Consequently, the requisite water retention and hydraulic conductivity functions used in the model may not apply in outer space.