  We use confocal microscopy to directly visualize the formation and complex
morphologies of trapped non-wetting fluid ganglia within a model 3D porous
medium. The wetting fluid continues to flow around the ganglia after they form;
this flow is characterized by a capillary number, Ca. We find that the ganglia
configurations do not vary for small Ca; by contrast, as Ca is increased above
a threshold value, the largest ganglia start to become mobilized and are
ultimately removed from the medium. By combining our 3D visualization with
measurements of the bulk transport, we show that this behavior can be
quantitatively understood by balancing the viscous forces exerted on the
ganglia with the pore-scale capillary forces that keep them trapped within the
medium. Our work thus helps elucidate the fluid dynamics underlying the
mobilization of a trapped non-wetting fluid from a 3D porous medium.
