  Two-fluid interfaces in porous media, an example of driven disordered
systems, were studied by a real time three-dimensional imaging technique with
pore scale resolution for a less viscous fluid displacing a more viscous one.
With increasing flow rate the interface transforms from flat to fingers and
thence to droplets for both drainage and imbibition. The results compare and
contrast the effects of randomness, both physical (geometry of the pore space)
and chemical (wettability of the fluids), on the dynamical instability and
identify the origin of the pore-scale processes that govern them.
