Liquid behavior in partially saturated porous media under variable gravity

Plant growth in restricted volumes of porous material is of interest for advanced life support systems for the National Aeronautics and Space Administration's future space missions. Reduced gravity conditions may affect fluid behavior in partially saturated porous media, requiring special considerations for growth media selection and root module design to ensure reliable water, air, and nutrient supply. Evidence suggests that fluid displacement patterns become unstable and enhance phase entrapment in the absence of gravity, thereby modifying macroscopic transport properties essential for fluid management decisions. Parabolic flight experiments have shown that preferential flows may lead to phase (air or gas) entrapment that would affect gaseous diffusion, as illustrated by lattice Boltzmann simulations. In microgravity, unstable flow patterns and particle rearrangement introduce uncertainty associated with particulate root growth media. These findings suggest that future efforts toward designing porous media and plant root modules in reduced gravity should focus on engineered plant growth media with stable pore space and spatially segregated domains that support water and nutrient retention in addition to gas exchange.