Water Movement in Void Spaces in Porous Media Under Microgravity.

A plant growth system for crop production under microgravity is part of a life supporting system designed for long-duration space missions. In space, plant growth in soil requires an understanding of water movement in soil void spaces under microgravity. On earth, under 1G conditions, water movement is expressed with the Richards equation. Under the microgravity conditions, however, water movement in soil is still not well understood. The objective of this study was to observe how water movement in void spaces of porous media was affected by changes in gravity during recurring 20 seconds-parabolic flights. We designed a new experimental column in which spherical particles were fixed side by side to serve as an idealized soil particle arrangement. We used 30 mm diameter glass particles to avoid the influence of matric potential and observed water movement when water was injected from the bottom of the particles under microgravity. When water injection was stopped, the water was held and maintained in contact with the spherical particles at pressures lower than air pressure. Furthermore, the water content in void spaces and contact angle of water on the particles sensitively varied with gravity change.