A repeating pattern of granular plugs emerges when a tube filled with water and grains is slowly drained. The moving air-water meniscus bulldozes the grains ahead, and friction eventually causes the grains to jam, forcing the air to penetrate through the accumulated material. Once through to the other side, the air starts to pile up another plug, and the process repeats until the whole tube is clogged full of granular plugs separated by empty gaps.
Dumazer et al., Frictional Fluid Dynamics and Plug Formation in Multiphase Millifluidic Flow. Phys. Rev. Lett. 117, 028002 – Published 7 July 2016. Dumazer_PRL028002