Competing Gravitational and Viscous Effects in 3D Two-Phase Flow Investigated With a Table-Top Optical Scanner

We present experiments and theory describing the transition from viscosity-stabilized flow to gravitationally unstable fingering for two-phase in a cubic box, filled with synthetic porous medium. Observation is made possible by use of our newly developed table-top 3D scanner based on optical index matching laser-induced fluorescence, which described detail. In experiment, more dense, viscous fluid injected at fixed rate point source top cell displaces less viscous, dense fluid. observe stable invasion zone near inlet, increases size increasing rates, presents initially close hemispherical shape. At later times, front transits an mode regime. The occurs predicted critical radius, R c , corresponding zero combined gravitational pressure gradient.