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We present experiments and theory describing the transition fromviscosity-stabilized flow to gravitationally unstable fingering for two-phaseflow in a 3D synthetic porous medium. Observation is made possible by the useof our newly developed table-top 3D-scanner based on optical index matching andlaser-induced fluorescence, which is described in detail. In the experiment, amore dense, more viscous fluid injected at a fixed flow-rate from a pointsource at the top of the flow cell displaces a less viscous, less dense fluid.We observe a stable invasion zone near the inlet, which increases in size withincreasing flow rates, and presents initially a close to hemispherical shape.At later times, the invasion front transits to an unstable mode and a fingeringflow regime. The transition occurs at a predicted critical radius, Rc,corresponding to the zero of the combined viscous and gravitational pressuregradient.

Competing gravitational and viscous efects in 3D two-phase flow investigated with a table-top optical scanner