Microgravity Implications of Water Distribution on Oxygen Diffusion Pathways in Unsaturated Porous Media.

The Optimization of Root Zone Substrates research (http://www.sdl.usu.edu/programs/orzs) investigates the effect of reduced gravity on porous-media fluid management at the root-module and pore scale. Previous measurements of water distribution and fluid transport in porous media revealed unanticipated fluid behavior including altered water retention and fluid distribution. These alterations potentially modify diffusion pathways and represent a hypothesized limitation for plant growth that requires exchange of respiratory gases. Our objectives were to investigate the impact of reduced gravity on water retention and diffusive gas-transport within three different pore-sized aggregated media.  Combining measurements of water retention and effective oxygen diffusion offered a unique opportunity to study sample scale water configuration. The existence, quantity and tortuosity of continuous gas-filled pathways for diffusive transport were inferred from lumped snapshots of apparent pore scale configurations. Our findings show significantly reduced oxygen diffusion rates for draining conditions suggesting a shift in percolation threshold to higher air-filled porosity than thresholds found on Earth. Why this effect is manifested more particularly for smaller particle-sized (0.25 – 1 mm) media remains unexplained and is counter intuitive. Bond number comparisons suggest water held in larger pores would be more affected by gravity and therefore would have a greater impact on the signature of oxygen transport for larger particle-sized (1-2 mm) media. We suspect the reduced diffusion rates at identical air-filled porosities may be due to the generation and maintenance of a water-filled cross-section within the sample, which shifts the percolation threshold. A second International Space Station experiment is planned to confirm results from the first experiment and look for an explanation of reduced diffusion in the fine-textured media.