Morphological transitions in partially gas-fluidized granular mixtures.

Experiments were conducted to investigate pattern formation during the defluidization of a partially fluidized bimodal granular mixture. Partial fluidization occurs when the system is driven at gas velocities that are insufficient to fluidize all of the constituent particles. Over time, the granular mixture evolves into a variety of patterns depending on the concentrations of large and small particles and the gas velocity. We show how vertically oriented pipes, containing large particles, grow at the interface between the fluidized and static zones. The heterogeneities in the permeability field focus the flow, causing localized fluidization, which in turn localizes the sedimentation of the large particles segregating the system. We discuss how the interplay between heterogeneities in material properties, fluid flow and fluid induced deformation may be relevant to a variety of geological processes.