Water Imbibition in Porous Media Under Simulated Microgravity Conditions.

To understand the moisture behavior in porous media under various small gravity conditions, it is necessary to conduct experiments using such as a sounding rocket, a parabolic flight or a drop tower. However, it is difficult to conduct such soil physical experiments because of a short microgravity duration or a high cost. Using a 3D-clinostat, on the other hand, might be an alternative means to simulate a microgravity condition for plant and microbial experiments. The 3D-clinostat is a device that cancels gravity in all directions by randomly rotating a sample. Although the 3D-clinostat has long been used for plant experiments, its applicability to soil physics experiments has been unknown. In this study, the applicability of a clinostat for simulating water imbibition in porous media under microgravity was evaluated. When imbibition experiments for sand were carried out using a 3D-clinostat, with which a fast flow and a slow flow alternately appeared. Water flow in sand with a large hydraulic conductivity was influenced by changes in a gravity direction whereas in loam with a small hydraulic conductivity, the influence of a gravity direction became negligible to confirm the validity of Washburn's equation. A water imbibition rate using the 3D-clinostat was consistent with that of a horizontal water infiltration experiment. It is reported that the imbibition rate in the ISS under microgravity was smaller than that of a horizontal infiltration experiment on the ground. This type of reduction in the imbibition rate under microgravity was not be found in the experiments using the 3D-clinostat. Therefore, we concluded that the 3D-clinostat might not properly simulate physical properties of water altered by microgravity.