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
Time lapse movie of pattern formation in a drying particulate suspension. A mixture of silicon dioxide particles and water is spread out on a horizontal glass slide, and evaporation generates a flow from the center towards the edge of the droplet. Large particles become pinned to the substrate by the fluid meniscus, and act like obstacles to the flow. Gradually smaller particles become trapped by the meniscus, and deposits grow sideways into irregular stripes. Patterns like these can be seen on the sides of muddy cars on rainy days.
A labyrinth pattern self assembles as a granular suspension dries out. The material is sandwiched between to glass plates, and fluid is withdrawn through a tube and a hole in the centre of the top plate. Fingers of air sweep up the granular material – glass beads of 100 micrometer average diameter – and produce a simply connected branching labyrinth. The constant spacing between the branches represents a balance between friction and surface tension at the interface. The initial disc is 35 cm across.