Displacement patterns in granular packs under gravitational and viscous instabilities: A three-dimensional visualization study

Two-phase flow driven by coupled gravitational and viscous instabilities is widely encountered in many natural systems and engineering applications. The interfacial dynamics involved in such processes are highly complex and remain insufficiently explored experimentally in three dimensions (3D). In this study, a 3D visualization platform based on planar laser-induced fluorescence is developed to investigate the effects of injection rate and pore structure on immiscible displacement behavior in a non-wetting phase displacing a wetting phase under coupled gravitational–viscous instability conditions. We identify four distinct unstable displacement morphologies, namely, discrete buoyant droplet, slug, straight continuous fingering, and dendritic fingering cluster. We establish an experimental phase diagram of the displacement pattern in the space of capillary number and the Bond number. We also introduce a set of geometric descriptors to quantify morphological evolution and phase connectivity. This work reveals how the competition and coupling among capillary, viscous, and buoyancy forces govern fluid fragmentation, coalescence, and fingering development in 3D porous media. The findings provide mechanistic insights into multiphase flow instabilities and have direct implications for a number of subsurface applications.