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Controlled delivery of water to plant root zone remains a challenge in the exploration of plant growth under zero gravity, in which soil water retention characteristics are difficult to predict due to possible soil particle rearrangement and pore restructuring. This paper aims to provide an understanding of the effect of soil particle rearrangement on the water retention characteristics in zero gravity. A three-dimensional pore network model is developed to predict water retention characteristics under two forms of particle packing—cubic packing (most porous) and rhombohedral packing (most compact). The pore-throat distributions, as input parameters to the network model, are derived from particle size distribution under the two forms of particle arrangement. The model simulations reveal the quantitative variation of water retention characteristics due to particle rearrangement. Soil particle rearrangement that occurs during the drying process is shown to significantly change the water retention curve when a soil is near saturation. When the soil is near residual water content, the particle rearrangement has little effect on the water retention curve. The model results indicated that an increase in packing density results in decreased water content at saturation, increased bubbling pressure, and accentuated water retention hysteresis. The model also reveals nonuniform spatial distribution of pore fluid in porous medium in zero gravity. Although this paper studied the soil-water characteristics from the microgravity perspective, its results also contribute to the understanding of the soil-water characteristics due to soil compaction on Earth. [ABSTRACT FROM AUTHOR]

Designing a reliable plant growth system for crop production in space requires the understanding of pore fled distribution in porous media under microgravity. The objective of this experimental investigation, which was conduced aboard NASA KC-135 reduced gravity flight, is to study possible particle separation and the distribution of discontinuous wetting fluid in porous media under microgravity. KC-135 aircraft prodded gravity conditions of 1, 1.8, and 10-2 g. Glass beads of a known size distribution were used as porous media; and Hexadecane, a petroleum compound immiscible with and lighter than water, was used as wetting fluid at residual saturation. Nitrogen freezer was used to solidify the discontinuous Hexadecane ganglia in glass beads to preserve the ganglia size changes during Afferent gravity conditions, so that the blob-size distributions (BSDs) could be measured after flight. It was concluded from this study that microgravity has little effect on the size distribution of pore fluid blobs corresponding to residual saturation of wetting fluids in porous media. The blobs showed no noticeable breakup or coalescence during microgravity. However, based on the increase in bulk volume of samples due to particle separation under microgravity, groups of particles, within which pore fluid blobs were encapsuled, appeared to have rearranged themselves under microgravity. [ABSTRACT FROM AUTHOR]

Understanding the effect of gravity on hydraulic properties of plant growth medium is essential for growing plants in space. The suitability of existing models to simulate hydraulic properties of porous medium is uncertain due to limited understanding of fundamental mechanisms controlling water and air transport in microgravity. The objective of this research was to characterize saturated and unsaturated hydraulic conductivity (K) of two particle-size distributions of baked ceramic aggregate using direct measurement techniques compatible with microgravity. Steady state (Method A) and instantaneous profile measurement (Method B) methods for K were used in a single experimental unit with horizontal flow through thin sections of porous medium providing an earth-based analog to microgravity. Comparison between methods was conducted using a crossover experimental design compatible with limited resources of space flight. Satiated (natural saturation) K ranged from 0.09 to 0.12 cm s-1 and 0.5 to >1 cm s-1 for 0.25- to 1- and 1- to 2-mm media, respectively. The K at the interaggregate/intraaggregate transition was ≈10-4 cm s-1 for both particle-size distributions. Significant differences in log10K due to method and porous medium were less than one order of magnitude and were attributed to variability in air entrapment. The van Genuchten/Mualem parametric models provided an adequate prediction of K of the interaggregate pore space, using residual water content for that pore space. The instantaneous profile method covers the range of water contents relevant to plant growth using fewer resources than Method A, all advantages for space flight where mass, volume, and astronaut time are limited. [ABSTRACT FROM AUTHOR]

Liquid behavior under reduced gravity conditions is of considerable interest for various components of life-support systems required for manned space missions. High costs and limited opportunities for spaceflight experiments hinder advances in reliable design and operation of elements involving fluids in unsaturated porous media such as plant growth facilities. We used parabolic flight experiments to characterize hydraulic properties under variable gravity conditions deduced from variations in matric potential over a range of water contents. We designed and tested novel measurement cells that allowed dynamic control of water content. Embedded time domain reflectometry probes and fast-responding tensiometers measured changes in water content and matric potential. For near-saturated conditions, we observed rapid establishment of equilibrium matric potentials during the recurring 20-s periods of microgravity. As media water content decreased, the concurrent decrease in hydraulic diffusivity resulted in limited attainment of equilibrium distributions of water content and matric potential in microgravity, and water content heterogeneity within the sample was influenced by the preceding hypergravity phase. For steady fluxes through saturated columns, we observed linear and constant hydraulic gradients during variable gravity, yielding saturated hydraulic conductivities similar to values measured under terrestrial gravity. Our results suggest that water distribution and retention behavior are sensitive to varied gravitational forces, whereas saturated hydraulic conductivity appears to be unaffected. Comparisons between measurements and simulations based on the Richards equation were in reasonable agreement, suggesting that fundamental laws of fluid flow and distribution for macroscopic transport derived on Earth are also applicable in microgravity. [ABSTRACT FROM AUTHOR]