Fingering Instability in Granular Suspension Injected into Thin Fractures*

We discuss observation of a fingering instability when granular beads suspended in a liquid, with the same mass density, are injected into a fracture in the form of a Hele-Shaw cell filled with a similar wetting fluid. When the suspension is injected from a point into the cell, with the same density fluid, we find that the suspension spreads uniformly with a smooth circular front, with an expansion rate proportional to the injection rate. In contrast, when the density of the injected fluid and the interstitial fluid are even slightly different, we find that fingers form when the granular suspension is injected, and as the front spreads above a critical radius. We demonstrate that only the density difference is important, and the patterns form with both higher and lower density interstitial fluid. We will discuss the evolution of the fingers as a function of injection rate and the volume fraction of the beads, and contrast with the Saffman-Taylor instability observed when a less viscous fluid is injected into a more viscous fluid.