The rheology of drag reducing cationic surfactant solutions was investigated using a controlled-stress rheometer. Both steady and oscillatory shear measurements were carried out at 25°C with the help of the cylindrical double-gap measuring geometry. The cationic surfactant investigated was Ethoquad T/13-27W, a commercial product manufactured by Akzo Nobel Chemicals Inc., U.S.A. Sodium salicylate (NaSal) was used as counterion. The effects of the surfactant and counterion concentrations on the rheological behaviour were also determined.

The surfactant solutions in the presence of counterion exhibit non-Newtonian rheological behaviour. At shear rates below some critical value, the surfactant solutions behave as either shear-thinning or Newtonian depending upon the counterion concentration; at low NaSal concentrations, shear-thinning is observed whereas at high NaSal concentrations, Newtonian behaviour is observed. When the shear rate is increased above the critical value, a rise in viscosity is observed indicating shear-thickening or dilatant behaviour. Oscillatory shear measurements indicate that the surfactant solutions are significantly viscoelastic. At a given surfactant concentration, the rheological parameters such as zero shear viscosity, critical shear rate, storage and loss moduli, etc. first increase, reach a maximum value, and then decrease with the increase in NaSal concentration. With the increase in surfactant concentration at a fixed NaSal/surfactant weight ratio, the rheological parameters show a significant increase.

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