Visualization and characterization of capillary-to-viscous transition in fluid-induced granular deformation

Comprehending pattern transitions during immiscible two-phase flow through granular materials is of paramount scientific and practical importance, as flow conditions are susceptible to variability along transport pathways in natural and engineering systems. The localized fluid displacement frequently mobilizes the hosting grains and triggers diverse morphological deformation. The interplay between such hydrodynamic and mechanical forces renders the deformation transitions inherently complex and hard to predict. Here, we integrate laboratory experiments with numerical simulations to probe the morphodynamics of fluid-driven granular deformation at the fundamental pore scale. Upon systematically varying capillary–viscous flows, we unveil three deformation patterns and propose a new dimensionless number incorporating the overlooked inertial and size effects to capture their transitions from slender channels to circular cavities to ramified fingers. We consistently observe lower residual saturation in the cavity regime attributed to the balanced capillary–viscous forces in this transition zone. Notably, our numerical model accurately mirrors the experimental observations and sheds light on the physics underlying the nucleation of characteristic crossover cavitation based on the microscale quantities of trivial energy leftover and uniform flow resistance.