Compaction and size segregation in a liquid-saturated grain packing due to pulsation effect during air injection

Injecting air into two-dimensional vertical liquid-saturated assemblies of grains causes a rearrangement of the grains. The interaction of the air flow injected at the bottom with the grains and the liquid leads to a mobilization of the grains, in which air channels migrate and grain clusters undergo shearing. The channel migration comes to a stop after some time, leaving one thin and stable preferential channel for air flow. Furthermore, the grain packing is compacted due to a rearrangement process caused by the pulsating movement of air channels. The compaction process is found to obey a slow exponential growth law. Additionally, the pulsation introduces size segregation in the packing. This is visualized by a set of tracing experiments showing that the coarser grains tend to accumulate at the downstream end of the preferential air pathway. However, the pulsating strength decreases sharply as the grain size increases. Therefore no segregation was observed in the coarse packing. A stabilization process of the preferential air channel can be described by a lower bound of critical channel size assuming the validity of Hagen–Poiseuille air flow inside the channel. Nevertheless, the channel size could not exceed an upper size which is determined by the capillary instability, assuming a quasi-static equilibrium of the dilating process of the air channel.