Abstract:
We present an experimental study and a numerical simulation of the effect of time-independent, localized perturbations applied to the interface in the Saffman-Taylor fingering problem.  When the perturbation is applied at a specific spot near the tip of the finger, the selection of the steady-state shape is drastically changed.  In particular, one can obtain fingers with a width well below  lambda=1/2.  A perturbation applied far away from the tip has no effect.  We observe the same behavior in the simulation and the experiment.