Pore-Scale Wetting Process of Capillary-Driven Flow in Unsaturated Porous Media under Micro- and Earth-Gravities

Microgravity hinders capillary-driven water flow in unsaturated porous media. Previous studies proposed pore-scale phenomena such as “air entrapment”, “particle separation”, and “interruption on widening void space” to explain gravity-dependent capillary-driven flows. Our objectives were: (1) to measure the water flux densities of the pore-scale capillary-driven flow in micro-and Earth-gravities and (2) to reveal that what makes water flow slower under microgravity than under 1 G. We found that average macroscopic water flux densities had no significant difference under micro-and Earth-gravities (p = 0.30). We did not observe “air entrapment” in the pore spaces of porous media. “Widening on a single particle” and “capillary widening” disturbed capillary-driven flow; however, “widening on a single particle” had no significant gravity dependency. “Capillary widening” may be independent of gravity, since it was observed both under microgravity and under 1 G. Water flux densities in unsaturated porous media may have gravity dependency induced by “particle separation” only when porosity is large enough to allow particles to move.