Segregation in Granular Flow Under Lunar Conditions

Every proposed lunar in situ resource utilization (ISRU) project, from oxygen extraction by hydrogen reduction or molten regolith electrolysis to polar water recovery, begins with bulk transport of granular lunar soil. Such mixtures are expected to segregate, and segregation is the largest source of feedstock non-uniformity in terrestrial processing. Whether that transfers to the Moon is not obvious: lunar regolith is fine, angular, agglutinate-bearing and electrostatically active, and is handled in vacuum at one-sixth gravity, shifting the cohesive–gravitational balance on which classical segregation theory implicitly rests. We ask which size and density segregation mechanisms survive that transition. We argue that the appropriate organizing variable is not gravity but the granular Bond number, and that reduced gravity may displace regolith handling toward a cohesion-dominated regime in which agglomerate- and charge-mediated separation may replace terrestrial kinetic sieving. Recent low-gravity hopper and drop-tower experiments show that, for cohesive discharge, clogging becomes more likely at reduced gravity and the data are organized by a granular Bond number rather than by gravity alone; whether the same holds for segregation has not been tested. We present a conceptual regime map in the granular Bond number–inertial number plane, separating where terrestrial correlations are expected to apply from where they may not, and derive seven testable hypotheses. No lunar-surface segregation experiment has yet been performed on a flowing mixture; we close with a prioritized research agenda and the minimum instrumentation such a measurement would require.