Reproducibility of the finger pattern in viscous fingering.

The complex pattern of viscous fingering (VF) appears to be chaotic. Its early evolution seems predictable but, on a long time-scale, it is not so and the transition is complex. Detailed experimental observations of fingering systems have been hindered by various limitations. We present a new method for visualising VF in particulate beds. Our results show that the onset of VF and its initial evolution are reasonably reproducible at very low Reynolds numbers (Re < 0.005). The transition to irreproducibility of the fingering pattern develops progressively over long migration distances. When the flow velocity increases, changes in the finger distribution take place over shorter migration distances, become more important, and the fingers evolve faster. Understanding this new aspect might allow improvements in the efficiency of processes governed by VF, e.g. injection of large, concentrated samples or transfer of large fractions between two streams having different viscosities.