Strongly driven granular media are known to undergo a transition from a gas-like to a cluster regime when the density of particles is increased. However, the main mechanism triggering this transition is not fully understood so far. Here, we investigate experimentally this transition within a 3D cell filled with beads that are driven by two face-to-face vibrating pistons in low gravity during parabolic flight campaigns. By varying large ranges of parameters, we obtain the full phase diagram of the dynamical regimes reached by the out-of-equilibrium system: gas, cluster or bouncing aggregate. The images of the cell recorded by two perpendicular cameras are processed to obtain the profiles of particle density along the vibration axis of the cell. A statistical test is then performed on these distributions to determinate which regime is reached by the system. The experimental results are found in very good agreement with theoretical models for the gas-cluster transition and for the emergence of the bouncing state. The transition is shown to occur when the typical propagation time needed to transmit the kinetic energy from one piston to the other is of the order of the relaxation time due to dissipative collisions.

Space exploration and exploitation face a major challenge: the handling of granular materials in low-gravity environments. Indeed, grains behave quite differently in space than on Earth, and the dissipative nature of the collisions between solid particles leads to clustering. Within poly-disperse materials, the question of segregation is highly relevant but has not been addressed so far in microgravity. From parabolic flight experiments on dilute binary granular media, we show that clustering can trigger a segregation mechanism, and we observe, for the first time, the formation of layered structures in the bulk. During the past decades space exploration and exploitation has remained in the spotlight of the scientific community and industry. Indeed, landing probes on distant planetoids, extracting rare earth elements on asteroids and 3D printing infrastructures on the Moon is as exciting as it could be lucrative.1 However, difficulties arise when it comes to the handling of regolith, the granular materials present on the surface of dusty celestial bodies.2 The physical ingredients of granular physics in space are straightforward: dissipative collisions, cohesion and electrostatic interactions for fine grains. For inelastic particles, i.e. with only dissipative collisions, a clustering of the granular material can occur. The latter phenomenon has been studied in microgravity since the nineties and was observed experimentally for the first time during a sounding rocket mission.3 Since then, many numerical and experimental studies in low gravity 4-7 were realised mainly with mono-disperse systems. However, recent numerical studies 8 indicate that the poly-dispersity of the granular media impacts strongly on its behaviour and may lead to segregation in zero g even though the usual mechanisms responsible for this phenomenon rely on the presence of gravity.Here, we present novel experimental results from the European Space Agency's parabolic flight campaign PFC64 exhibiting spectacular pattern formation within a binary granular system under microgravity conditions. Our experiments were performed within the framework of the VIP-Gran instrument, whose setup consists in a 45 × 30 × 5 mm 3 rectangular box in which two opposing walls (30 × 5 mm 2 ) act as pistons. They oscillate sinusoidally in anti phase along the longitudinal axis (45 mm) of the box with a typical amplitude of 3 mm and a typical frequency of 20 Hz. We studied four different granular loadings composed of N s small and N l large bronze beads with respective diameters d s = 1 mm and d l = 2 mm. Both species have respective masses m s = 4.8 mg and m l = 38.4 mg. The restitution coefficient for the different types of collisions were not measured experimentally. However, given the low grain velocities encountered in our experiment, a value of ε = 0.9 can be assumed for bronze-bronze interaction 9

A new experimental facility has been designed and constructed to study driven granular media in a low-gravity environment. This versatile instrument, fully automatized, with a modular design based on several interchangeable experimental cells, allows us to investigate research topics ranging from dilute to dense regimes of granular media such as granular gas, segregation, convection, sound propagation, jamming, and rheology-all without the disturbance by gravitational stresses active on Earth. Here, we present the main parameters, protocols, and performance characteristics of the instrument. The current scientific objectives are then briefly described and, as a proof of concept, some first selected results obtained in low gravity during parabolic flight campaigns are presented.

We investigate numerically and theoretically the internal structures of a driven granular gas in cuboidal cell geometries. Clustering is reported and particles are classified as gaseous or clustered via a local packing fraction criterion based on a Voronoi tessellation. We observe that small clusters arise in the corners of the box, elucidating early reports of partial clustering. These aggregates have a condensation-like surface growth. When a critical size is reached, a structural transition occurs and all clusters merge together, leaving a hole in the center of the cell. This hole then becomes the new center of particle capture. Taking into account all structural modifications and defining a saturation packing fraction, we propose an empirical model for the cluster growth.

In microgravity, the successive inelastic collisions in a granular gas can lead to a dynamical clustering of the particles. This transition depends on the filling fraction of the system, the restitution of the used materials and on the size of the particles. We report simulations of driven bi-disperse gas made of small and large spheres. The size as well as the mass difference imply a strong modification in the kinematic chain of collisions and therefore alter significantly the formation of a cluster. Moreover, the different dynamical behaviors can also lead to a demixing of the system, adding a few small particles in a gas of large ones can lead to a partial clustering of the taller type. We realized a detailed phase diagram recovering the encountered regimes and developed a theoretical model predicting the possibility of dynamical clustering in binary systems.