When grains are continuously driven in a closed cell, inelastic collisions may lead the system into a dense cluster which coexists with a granular gas. Although observed 12 years ago in microgravity experiments , the dynamical clustering of grains was still puzzling. Moreover, a recent numerical study  emphasized that the physical conditions for clustering, as well as the related dynamical regimes, are much more complicated than early proposed.
In a new study , we propose a model for describing the dynamical clustering of particles. The transition between a granular gas and a liquid-like phase is in fact linked to cascades of inelastic collisions in the system. Our model was tested in numerical simulations (within the VIPGRAN instrument of the ESA project ) and compared to earlier experiments in microgravity . This model fits the data without any free parameter ! See below the excellent agreement between the model (in red) and the data in a diagram where represents the dimensionless mean free path of a single particle and represents the volume fraction in the cell.
Many fundamental questions remain about the time evolution of the cluster with respect to the renewal of components, the definition of the cluster boundary or the possible existence of different cluster types. This model will help scientists to select the right experimental conditions in order to investigate new physical phenomena in space. The cluster-gas transition represents indeed the focus point of a Work Package (more precisely WP1) in the VIPGRAN project.
 E. Falcon, R. Wunenburger, P. Evesque, S. Fauve, C. Chabot, Y. Garrabos and D. Beysens, Phys. Rev. Lett. 83, 2, 440 (1999)
 E.Opsomer, F.Ludewig, and N.Vandewalle, Phys. Rev. E 84, 051306 (2011) – PDF
 E.Opsomer, F.Ludewig, and N.Vandewalle, EPL 99, 40001 (2012) – PDF
 See the web pages of the VIPGRAN project.