VISCOUS FINGER WIDENING WITH SURFACTANTS AND POLYMERS

We study the viscous fingering instability which results from a competition between capillary and viscous forces. We show that for two different systems the instability is modified drastically by changing the surface tension or viscosity by means of surfactants or polymers. For both systems the width of the finger can increase with increasing velocity before settling at a plateau value larger than half the channel width. A numerical study shows that the large deviations from the classical result can be attributed to a velocity dependence of the dynamic interfacial tension and viscosity.