Granular clogging across gravities: a unified scaling

TL;DR

This study introduces a universal scaling law for granular flow clogging across different gravitational environments by incorporating the granular Bond number, enabling accurate predictions of clogging behavior in low gravity conditions relevant to space missions.

Contribution

The paper identifies the granular Bond number as a key control parameter and demonstrates its effectiveness in unifying granular clogging behavior across varying gravitational accelerations.

Findings

Clogging probability increases in low gravity environments.

Rescaling data with the Bond number collapses diverse results into a unified diagram.

Experimental validation with extraterrestrial soil simulants confirms the model's predictions.

Abstract

Lacking a universal law for granular flows across gravitational environments, fundamental processes such as hopper discharge remain vulnerable to failure in low gravity environments. A central challenge is clogging, the spontaneous arrest of flow through a constriction; yet previous studies report contradictory results on its dependence on gravitational acceleration. We identify the granular Bond number as the missing control parameter, defined as the ratio of intrinsic cohesive interactions among particles to gravity. Based on an in-bulk measurement of this quantity, we propose to rescale Earth-measured data for predicting granular behavior in low gravity. We present experiments of granular flow through an orifice under true reduced gravity (Moon and Mars), using an active drop tower, and extraterrestrial soil simulants as model cohesive materials. Our experiments reveal substantially increases in clogging probability, contrary to previously predicted, which depends on the properties of the material itself. When rescaled by the Bond number, seemingly conflicting results can be explained and collapse into a unified state diagram, predicting clogging across materials and gravitational accelerations. This establishes a general framework for cohesion--gravity competition. Future space missions to the Moon, Mars, and asteroids will rely on such predictions of granular behavior in low gravity.