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

www.nature.com/npjmgrav ARTICLE OPEN Granular piston-probing in microgravity: powder compression, from densification to jamming Olfa D’Angelo 1,2 ✉ , Anabelle Horb3,5, Aidan Cowley3, Matthias Sperl 1,4 and W. Till Kranz1,4 1234567890():,; 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 (φJμ
g ¼ 0:567 ± 0:014) than on-ground (φgnd ¼ 0:579 ± 0:014). J 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. npj Microgravity (2022)8:48 ; https://doi.org/10.1038/s41526-022-00235-2 INTRODUCTION As humans make their way into space and plan to travel to sandcovered celestial bodies, the influence of (reduced) gravity on granular phenomena becomes a matter of concern1, and new strategies are sought to handle granular materials efficiently in various gravitational environments2–5. While the action of gravity is intuitively expected to weaken as objects become smaller, the collective behavior of small particles composing granular systems is significantly modified by the gravitational field, challenging our expectation of powder response outside of Earth’s gravity. Granular media are by definition composed of grains big enough not to be subject to thermal motion6,7 but small enough for continuous material properties to emerge on the mesoscopic scale. Such grains are, therefore, generally sufficiently large for gravitational forces on each of them not to be negligible in comparison to the other forces in present. The flow behavior of granular matter can thus change dramatically with a change in gravitational acceleration, such as that affecting a body on a foreign celestial surface (e.g. Moon, Mars, or an asteroid), or subjected to microgravity conditions. The aim of this article is to investigate the densification of a granular packing in two scenarios: on-ground (gnd), where Earth’s gravity may accelerate particles relative to the fixed container, and in microgravity (μ-g), on parabolic flights, where gravity cannot induce such relative acceleration. Note, that the term microgravity is widely used, although free-fall would be more precise8. We propose a densification mechanism in the form of a piston rising quasi-statically through a confined powder bed (see Fig. 1), thereby compressing the material placed above. The piston is of diameter inferior to that of the container. Compression is continued until reaching a jammed state above the piston, hence probing the jamming point variations between the two environments. The jammed state is characterized by the appearance of a finite elastic modulus of the granular packing, which drives the piston to a stop. Such piston mechanism could be used, onground or in space, to densify shallow granular beds and probe the percolation of the force network, possibly before submitting the material to further processing. In presence of gravity, granular flows can be caused by their potential energy in gravity-driven flows, providing a granular time scale in the form of the time needed for grain to fall by its diameter. In microgravity, particles do not fall towards the minimum-height position available; instead, they cluster into floating aggregates9–12, as the inter-particle cohesive forces become the predominant interaction. If no force field sediments the packing, particles or clusters are not necessarily in contact with their closest neighbors13. Force chains14,15, enabling force transmission through lasting contacts in a dense granular medium, cannot appear unless a contact network is established. Yet, such contact network can also trigger flow arrest on the macroscopic scale, if it percolates into a force network spanning the system, transmitting force to its boundaries akin to a solid16–18. The jamming transition19,20 describes the changeover between the flowing state, in which the granular fabric deforms plastically, and the jammed state, where a mechanically stable contact network is formed, which can withstand a certain amount of stress without reorganizing. In other words, the granular assembly as a whole exhibits a finite elastic modulus. If unpredicted, the appearance of jammed regions can be detrimental to industrial processes involving handling and transport of granular materials, as it can draw entire production processes to a halt. In sheared granular media, an anisotropic shear-jammed phase can block powder flow mono-directionally21–24. It was suggested that for frictionless particles, this transition already occurs at random loose packing (rlp)25,26, while a granulate in the state of random close 1 Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR), 51170 Köln, Germany. 2Institute for Multiscale Simulation, Universität ErlangenNürnberg, Cauerstraße 3, 91058 Erlangen, Germany. 3European Astronaut Centre (EAC), European Space Agency (ESA), 51170 Köln, Germany. 4Institut für Theoretische Physik, Universität zu Köln, 50937 Köln, Germany. 5Present address: Omnidea, Lda. – Polypark, Nùcleo Empresarial da Arruda dos Vinhos, Estrada da Quinta de Matos 4, 2630-179 Arruda dos Vinhos, Portugal. ✉email: Published in cooperation with the Biodesign Institute at Arizona State University, with the support of NASA O. D’Angelo et al. 2 1234567890():,; packing would be isotropically jammed27,28. However, the existence of a fixed jamming packing fraction φJ (jamming point) has now been challenged29,30. Besides evident dependence on particles’ shape31,32, (poly) dispersity33 or mechanical properties32, experimental, and numerical studies have shown the history-dependence of φJ, with variations, even for a single granular material, related to the preparation protocol29–31,34,35 or compression rate of the granular bed leading to jamming (namely, φJ decreases with increasing compression rate33,36). Interparticular friction was shown to decrease the jamming packing by decreasing part (...truncated)