Liquid-Gas Interfacial Configurations in Angular Pores under Microgravity

Plant growth in limited volumes of porous substrates is of interest for advanced life support systems for NASA's future space missions. Liquid behavior under microgravity introduces special considerations for selection of suitable porous plant growth media and the design of root modules for reliable water, air, and nutrient supply. It has recently become evident that the macroscopic dynamic behavior of the temporal and spatial under-saturation levels in root modules strongly depends on the physical behavior of the fluid at the microscopic (pore-scale) level. Ultimately, stability and break-up of liquid-bridges, -columns, and -films, confounded by, contact angle dynamics and hysteresis, phase discontinuities, capillary fingering and entrapment of the non-wetting phase together with dynamic rearrangement of solid particles and the complex geometry of porous media determine the saturation levels and its stability at macroscopic scales. However, it is not yet clear if and to what extent these micro-scale phenomena are gravity dependent. As a first step in studying liquid configurations in partially saturated angular pores we employed the Augmented Young-Laplace equation that simultaneously considers the contributions of capillary and adsorptive forces with a variable gravity term. Interfacial configurations in microgravity compared to gravity conditions for pores with various sizes and angularities. The difference between conditions for liquid held between parallel plates become significant only for pores larger than approximately 1 mm and for potentials very close to zero. Anecdotal evidence suggests that for Bond Number > 0.05 water flow under 1g becomes unstable (corresponding to flow in pores with radius > 0.6 mm. For smaller pores and for lower ambient matric potentials, the shape of liquid-gas interface at 1 and 0g are indistinguishable. Implications of liquid configurations on gas entrapment processes and on macroscopic retention and transport properties relevant to root module design are discussed.