The finger-like branching pattern that occurs when a less viscous fluid displaces a more viscous one confined between two parallel plates, the so-called Hele-Shaw geometry, has been studied widely and with various normal fluids. We have investigated the granular analogue of such Hele–Shaw cell, where air (the low-viscosity fluid) displaces glass beads (the high viscosity granular "fluid"). Because granular fluids composed of dry, non-cohesive grains exhibit negligible surface tension, this allows us to explore a regime not accessible with ordinary fluids. We demonstrate that the grain–gas interface exhibits a fractal structure and sharp cusps, which are associated with the hitherto-unrealizable singular hydrodynamics predicted for the zero-surface-tension limit of normal fluid fingering. The scaling for the finger width is distinct from that for ordinary fluids, reflecting unique granular properties such as friction-induced dissipation as opposed to viscous damping. However, the fractal dimension of the fingering pattern and the shape of the singular cusps on the interface agree with the theories based on simple Laplacian growth of conventional fluid fingering in the zero-surface-tension limit. Read more at Nature News.

1. •Xiang Cheng, Lei Xu, Aaron Patterson, Heinrich M. Jaeger and Sidney Nagel, "Towards the zero-surface-tension limit in granular fingering instability", Nature Physics 4, 234 (2008). pdf