Controlling singularities in the complex plane: Experiments in real space

The theoretical interpretation of the selection in a linear channel of the Saffman-Taylor finger of width {λ}=0.5 involves singularities of the zero-surface-tension analytical solutions in the complex plane. In such a physical situation the complex plane can be mapped onto the liquid region in real space. Experiments are presented showing that by a very local disturbance of the viscous flow the observed finger width is changed. The results are in quantitative agreement with the theory; the flow disturbance induces a modification of the singularities in the complex plane and thus changes the finger-width selection.