Push air into a gap filled with a viscous fluid, and you’ll get the branching, dendritic pattern of a Saffman-Taylor instability. Here, researchers use a similar set-up: injection into a narrow gap between transparent planes to explore something quite different. In this experiment, the gap was initially filled with a mixture of air and tiny hydrophobic glass beads. When the team injected a viscous mixture of water and glycerol, new patterns emerged. At low injection rates, a single finger structure formed. But at high injection rates, a whole spoke-like pattern formed. (Image and research credit: D. Zhang et al.; via Physics Today)
Granular Gaps
