No abstract

We report an experimental study of a dilute “gas” of inelastically colliding particles excited by vibrations in low gravity. We show that recording the collision frequency together with the impulses on a wall of the container gives access to several quantities of interest. We observe that the mean collision frequency does not scale linearly with the number N of particles in the container. This is due to the dissipative nature of the collisions and is also directly related to the non-extensive behaviour of the kinetic energy (the granular temperature is not intensive).

Light transmission measurements performed in SF 6 close to its liquidgas critical point are used to obtain turbidity data in the reduced temperature rangeAutomatic experiments (ALICE 2 facility) were made at a near critical density, i.e., ρ −ρ c ρ c = 0.8 %, in the one-phase homogeneous region, under the microgravity environment of the Mir Space Station ( ρ is the average density, ρ c is the critical density). The turbidity data analysis verifies the theoretical crossover formulations for the isothermal compressibility κ T and the correlation length ξ . These latter formulations are also used to analyze very near T c thermal diffusivity data obtained under microgravity conditions by Wilkinson et al. (Phys. Rev. E 57, 436, 1998).

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.