Gas Compression Impact on Stabilizing Deformation of Wet Granular Media

Gas invasion into liquid-filled granular media from a confined geometry is a prevalent phenomenon in geological and environmental applications such as underground carbon storage and air sparging. This gas-liquid displacement represents one of the most fundamental and common scenarios of two-phase flows. In addition to the typically negligible viscosities, gases are also much more compressible than liquids; as a consequence, the volumetric gas compression will lead to fluctuations in actual flow rates, thereby exerting a substantial influence on displacement processes and pattern formation. Despite this, previous experimental and theoretical studies have usually simplified both fluids as incompressible, leaving the role of compressibility underexplored. Moreover, the localized fluid-fluid displacement within the granular skeleton frequently mobilizes the host grains, resulting in diverse morphological deformation. Here, we experimentally investigate the effects of gas compressibility on the air-induced deformation of oil-saturated granular media confined in a Hele-Shaw cell. Specifically, we inject air into a monolayer of mobile grains that are saturated with silicone oil, maintaining a constant nominal flow rate while varying the initial air syringe volumes. We discover that greater compression, achieved with larger syringes, has a stabilizing effect on the gas-liquid-grain interface, shifting it from unstable dendritic/ramified fingers to more circular, less branched patterns. The reduced severity of the fingering deformation stems from the decreased compressive pressurization in the gas, which counteracts the viscous depressurization in the displaced liquid. Additionally, we observe that the injection pressure exhibits a non-monotonic evolution, with peak values notably delayed under higher compression. Our findings underscore the role of gas compressibility—neglected in laboratories yet becoming increasingly critical towards industrial relevance.