Capturing gas in soft granular media.

Gas migration through a soft granular material involves a strong coupling between the motion of the gas and the deformation of the material. We study this process experimentally by injecting air into a quasi-2D packing of soft particles and measuring the morphology of the air as it rises due to buoyancy. We systematically increase the confining pre-stress in the packing by compressing it with a fluid-permeable piston, leading to a gradual transition in migration mechanism from fluidization to pathway opening to pore invasion. By connecting these mechanisms quantitively with macroscopic invasion, trapping, and venting, we show that the mixed-mode transitional regime enables a sharp increase in the amount of gas trapped within the packing, as well as much larger venting events. We report our experimental findings and present a simple mechanistic model to rationalize our observations.