fig. 1.Snapshot sequences of typical experiments. Starting with an initially small
height difference, after a while, this height difference grows faster and faster until
one of the branches empties. (Top) Tube with smooth walls. The granular materials
appear black. (Bottom) Tube with walls partially covered with sandpaper, to increase
wall roughness. The sandpaper covers almost all the inner walls, except for a small
vertical strip, to see the height of the granular columns. Both sandpaper and grains
appear black.