Granular piston-probing in microgravity: powder compression, from densification to jamming

The macroscopic response of granular solids is determined by the microscopic fabric of force chains, which, in turn, is intimately linked to the history of the solid. To query the influence of gravity on powder flow behavior, a granular material is subjected to compression by a piston in a closed container, on-ground and in microgravity (on parabolic flights). Results show that piston-probing densifies the packing, eventually leading to jamming of the material compressed by the piston, regardless of the gravitational environment. The onset of jamming is found to appear at lower packing fraction in microgravity (phi(mu-g)(J) = 0.567 +/- 0.014) than on-ground (phi(gnd)(j) = 0.579 +/- 0.014). We interpret these findings as the manifestation of a granular fabric altered by the gravitational force field: in absence of a secondary load (due to gravitational acceleration) to stimulate reorganization in a different direction to the major compression stress, the particles' configuration becomes stable at lower density, as the particles have no external drive to promote reorganization into a denser packing. This is coupled with a change in interparticular force balance which takes place under low gravity, as cohesive interactions become predominant. We propose a combination of microscopic and continuum arguments to rationalize our results.

Automated summary

The macroscopic response of granular solids is closely related to the microscopic fabric of force chains and the history of the solid. The influence of gravity on powder flow behavior is studied by compressing a granular material in a closed container using a piston, both on the ground and in microgravity. The results show that jamming of the material occurs regardless of the gravitational environment, but the onset of jamming happens at a lower packing fraction in microgravity compared to on the ground. This is attributed to the alteration of the granular fabric by the gravitational force field, leading to a stable particle configuration at a lower density in the absence of a secondary load.