Viscous Fingering in Surfactant Solutions in a Hele-Shaw Cell

Viscous fingering in surfactant solutions in a rectangular Hele-Shaw cell was studied. The test fluids used for more viscous fluid were aqueous solutions of CIAB (Hexadecyltrimethylammonium Bomide) with NaSal as salt. The concentration ratio of CTAB to NaSal was 1:7.7 and two fluids with different CTAB concentration i.e. 0.03 mol/l and 0.04 mol/l were employed. Air, less viscous fluid, was injected into the more viscous fluid at various injection pressures. The finger growth changed at a critical wall shear rate: sudden jumping out of a finger-tip was observed. The order of the shear rate is similar to that at which flow curves bend in measurements with a capirally rheometer. The drastic change in finger growth relates to anomalous rheological property of the surfactant solution.