Granular Fingering in Fluid Injection into Dense Granular Media in a Hele-Shaw Cell

We study experimentally spontaneous pattern formation in a dry dense granular medium invaded by an aqueous glycerin solution in a radial Hele-Shaw cell. By varying the invading fluid viscosity via the weight concentration of glycerin, and by adjusting the normalized injection velocity via the injection rate and the gap size of the cell, we observe four distinct fluid-grain displacement regimes: (i) a simple radial flow regime, (ii) an infiltration-dominated regime, (iii) a grain displacement-dominated regime, and (iv) a viscous fingering-dominated regime. We argue that these displacement regimes emerge as a result of competition among the various energy dissipation mechanisms and can be classified based on the characteristic times involved in the injection process.