The Microgravity Effects on Upward Infiltration in Porous Media.

The Global Exploration Roadmap targets the fulfillment of Mars manned exploration by 2030s. In order to achieve a successful long-term space mission, durable and sustainable life supporting systems are required. A plant growth system for crop production under microgravity is part of a life supporting systems. In space, growing crops in soil culture requires the understanding of water behavior in porous media under various gravity conditions. On the earth, under the 1G condition, water flow is expressed by Darcy-Backingham’s equation, meaning that water moves in accordance with a matric potential gradient when there is no pressure and gravitational potentials. Under microgravity conditions, however, water behavior in porous media is still not well understood. Our recent research, which observed water behavior in void spaces, showed that water was retained in void spaces as thin water films and water stayed as it was during under microgravity. The current water flow theory based on Darcy-Backingham’s law under 1G may not be valid for water flow under microgravity. Porous media are assumed to be a bundle of capillary tubes with various diameters. We confirmed that water levels in vertically-installed capillary tubes with various diameters increased under microgravity just like described as the capillary rise theory. Under microgravity, on the one hand it seems likely that water hardly move in void spaces, but on the other hand capillary rise theory found to be true. Therefore, it would seem to be necessary to review water movement theories considering porous media as a bundle of capillary tubes. We compared water rises in a capillary tube and in porous media, both vertically installed on the water surface. The mean pore size of the porous media was same as the capillary tube diameter. The changes in the water levels of the porous media and the capillary tube were compared when gravity changes from 1G to μG. The results of this experiment will be discussed.