Experiments on perturbed Saffman-Taylor flows

We have performed pattern formation experiments where a relatively well understood system (flow in a Hele-Shaw cell) is perturbed either by means of a lattice of grooves or by the use of viscoelastic fluids. We have extended the qualitative analysis found in the literature for anisotropic fingering patterns, presenting a more quantitative approach that may prove useful as a tool to attack more complex problems. We have analyzed the different morphological regimes and looked for signatures of the transition between phases, with partial success when we try to characterize a given morphology quantitatively. In our studies of viscoelastic Hele-Shaw flow with associative polymer solutions, we have observed a transition from viscous fingering patterns into a regime where the growing patterns resemble the fracture in brittle solids. We have been able to rescale the threshold for these transitions, and we have observed interesting properties in a regime of fracturelike patterns where, under sorne circumstances, we have measured a characteristic oscillation frequency which shows interesting regularities. We have also studied the pressure in the viscoelastic flow, and found consistent results that may be used to implement a better theoretical model to fully understand the dynamics