Granular charge and discharge in low gravity

Granular flow models tend to fail in low gravity. When gravity decreases below a critical acceleration, cohesive forces become predominant, resulting in a shift in macroscopic behavior. Yet, these models are still used to prepare planetary exploration missions, for lack of better ones. For granular processing in space, essential for sustaining human presence on the Moon, this can have disastrous consequences.

We investigate the influence of partial gravity on granular flows, focusing on hopper discharge (granular discharge flow through an orifice). Using an hourglass with different aperture sizes and multiple materials, including regolith simulants, we perform experiments across a wide range of effective gravitational accelerations using drop tower and parabolic flight experiments. A centrifuge installed inside the aircraft allows us to generate any level of partial gravity in-flight.

We find that lunar gravity consistently increases the clogging probability and slows down the flow rate, which deviates from the scaling predicted by the Beverloo equation. We also find that tribocharging occurs, and its effect on flow can be isolated in reduced gravity. We propose a correction to the Beverloo equation and a clogging state diagram based on the granular Bond number: the cohesion-to-gravity ratio. Scaling the data with this parameter collapses scattered observations into a universal framework for predicting granular flow and clogging in low gravity.