Viscous fingers form when in a thin linear channel a fluid pushes a more viscous fluid. The instability of the interface results from a competition between viscous and capillary forces. We show here by acting on the viscosity or the surface tension by means of surfactants or polymers that the instability can be modified drastically. For the two different systems, unlike in the classical system, the width of the finger can go through a minimum and increases with increasing velocity before settling at a plateau value larger than half the channel width. A numerical resolution of the relevant hydrodynamic equations reveals that these large deviations from the classical result can be interpreted in terms of a velocity dependent dynamic interfacial tension for the surfactant system and viscosity for the polymer solution.
