Anomalous motion of viscous fingers in surfactant solutions in a Hele–Shaw cell

Viscous fingering in surfactant solutions in a rectangular Hele–Shaw cell was investigated. Test fluids were aqueous solutions of cetyltrimethylammonium bromide (CTAB) with sodium salicylate (NaSal) as a counter ion, and the ratio of mole concentration of CTAB and that of NaSal was 1–7.7. Two fluids that had a mole concentration different from that of CTAB were used. Air was injected into the cell and the growth of the interface between air and a CTAB/NaSal solution was observed. The fingertip grew similar to the finger growth in shear-thinning fluids at low pressure gradients. It took a cuspidate shape at the intermediate pressure gradient, and a sudden protrusion at a critical shear rate occurred. In high shear rate regions, the finger behaved as in a less shear-thinning fluid. These phenomena relate to rheological properties of the test fluids. Comparison with flow curves for CTAB/NaSal systems showed that the critical shear rate related to the shear rate at which a bending point appeared in the flow curve.