Abstract
Understanding water behavior in porous media in microgravity conditions is essential for growing soil-cultivated crops in outer space. A drop tower is a cost-effective method to generate microgravity environments. Still, it provides only a few seconds or less microgravity, which is insufficient for observing time-consuming phenomena such as infiltration in soil. In this study, we proposed a new infiltration procedure, where infiltration occurred repeatedly for 0.4 s μG conditions generated by a 2 m high drop tower. The infiltration for the next 0.4 s μG proceeded after a new wetting front location was initiated by manually advancing the wetting front. Repeating this procedure, referred to the intermittent infiltration procedure hereafter, enabled us to observe long-term infiltration under microgravity condition. We measured infiltration rates in glass beads of 0.4 mm and 2 mm diameter using the intermittent infiltration procedure. The infiltration rate for 0.4 mm glass beads agreed with that observed in the continuous infiltration experiment, although that for 2 mm glass beads was underestimated. We anticipated that this limitation could be overcome by utilizing higher drop towers capable of generating several seconds of microgravity. The intermittent infiltration procedure opened affordable measurements of soil physical parameters under microgravity to address challenges associated with repetitive experiments. Numerical simulations were also conducted to evaluate the intermittent infiltration, assuming porous media as a bundle of capillaries, by comparing infiltration rates between continuous infiltration over an extended period and intermittent infiltration by integrating 0.4-second infiltration. For capillaries with a 0.3 mm inner diameter, the infiltration rates for intermittent and continuous infiltration remained consistent. However, the inertial regime must be considered for capillaries with an inner diameter greater than 1.5 mm, leading to underestimating infiltration rates.
https://ror.org/02rqvrp93 
