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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]