Granular jamming and rheology in microgravity

TL;DR

The paper investigates how reduced gravity affects granular jamming and rheology by performing compression and Taylor-Couette–style shear tests in microgravity on two Bremen platforms and on Earth. By fluidizing, compressing to a fixed jam pressure, and measuring packing density via piston height, the authors determine that the jamming packing fraction satisfies $\varphi_\text{J}(g_\mu) < \varphi_\text{J}(g_E)$, while rheological measurements show higher average shear stress in microgravity, e.g., $\langle \tau \rangle_{g_\mu} > \langle \tau \rangle_{g_E}$, with stronger cohesion effects at low gravity. They interpret these results through the absence of gravity-driven secondary flows and the increased prominence of cohesive interparticle forces in microgravity, suggesting a gravity-dependent granular rheology that differs from Earth. The findings have practical implications for planetary exploration, construction, and regolith processing, and point to the need for generalized models that remain valid in low- and microgravity environments. Future work should extend duration, diversify particle properties, and explore longer-term dynamics to build robust low-gravity granular theories.

Abstract

Understanding how granular materials behave in low gravity is crucial for planetary science and space exploration. It can also help us understand granular phenomena usually hidden by gravity. On Earth, gravity dominates granular behavior, but disentangling its role from intrinsic particle interactions is challenging. We present a series of compression and shear experiments conducted in microgravity using the Center of Applied Space Technology and Microgravity (ZARM) drop tower and GraviTower Bremen (GTB). Our in-house developed experimental setup enables precise measurement of packing density and in-situ shear stress via a Taylor-Couette rheometer. We find that the jamming transition occurs at lower packing density in microgravity than on Earth, confirming that gravity promotes densification. Rheological measurements further reveal that in microgravity, the lack of a secondary force field and predominance of cohesive interparticle forces increase the stress needed for granular media to flow. These findings highlight gravity's dual role in enhancing both compaction and flow, and demonstrate the need for tailored granular models, valid in low- and microgravity environments.

Figures (17)

• Figure 1: Schematic of experimental setup. The basic experiments conducted consist of (1) compression and (2) shear of a granular material. Data recording devices are: an optical distance sensor (orange) to measure the piston's height, $h$; a rotary torque sensor (green) to measure shear stress, $\tau$; a normal load sensor (red) to estimate normal pressure changes, $\Delta\sigma$, due to the material's expansion or densification.
• Figure 2: Picture of experimental setup. (a) Upper and (b) bottom part of the setup. The full setup is placed inside a capsule to undergo microgravity experiments, in the drop tower or in the .
• Figure 3: Acceleration in the experimental capsule and corresponding experiment timeline. (a) Acceleration versus time during a experiment. Shear and compression experiments are conducted independently: either one or the other is conducted during each experiment. (b) Acceleration during a drop tower experiment with catapult. Compression, then shear are conducted consecutively. Note that the instant at which the jamming point is reached, $t_\text{J}$, is not time-controlled but controlled by the pressure imposed by the piston.
• Figure 4: Granular Bond number, $\mathbfcal{B}$, for increasing particle size. $\mathcal{B}$ is calculated, as defined in Eq. \ref{['eq:mynameisbond']}, for $g_E=9.81\m\per\square s$ and for $g_\mu =10^{-3} g_E$. Vertical grey lines indicate the tested particle diameters (dashed: $d=80µm$; dash-dotted: $d=1\mm$); a horizontal line at $\mathcal{B}=1$ marks the transition to cohesive behavior (above). The inset shows the force $F_\text{vdW}(r)$, and $F_\text{g}(r)$ for both $g_E$ and $g_\mu$. The main panel and inset have the same $x$-axis boundaries.
• Figure 5: Section view of the fluidization system. This assembly is placed at the bottom the experimental setup, as indicated in Fig. \ref{['fig:photosetup']}.