To Flow or Not To Flow? The granular Bond number to predict clogging in low gravity

In granular hopper flow, is the clogging probability influenced by gravity? Clogging, the spontaneous arrest of granular flow through a constriction, is a fundamental and widespread phenomenon in granular processing. Accurately predicting clogging risk is essential for space-related activities. Yet, conflicting results leave us without a reliable way to extrapolate terrestrial data to low gravity. We introduce an in-bulk definition of the dimensionless granular Bond number, B, to scale clogging probability from Earth gravity to low gravity. By testing multiple lunar regolith simulants in Earth and Moon gravitational accelerations, using an active drop tower, we reveal that low gravity significantly increases clogging risk, challenging previous findings. We observe a gravity-induced shift in the critical orifice below which clogging occurs by up to one order of magnitude. Scaling data with the Bond number unifies scattered results into a state diagram predicting clogging across materials and gravitational accelerations. Our findings provide a practical tool for anticipating granular clogging in reduced gravity, informing future space missions.