Under what conditions do we find drag-reducing surfactants

2 Under what conditions do we find drag-reducing surfactants  

The phenomenon occurs often in surfactant solutions in which small rod-like micelles are formed that are charged and the charge is not screened by excess salt [104]. Many surfactant systems have been found where such conditions exist. Typically the length of the rods is 

In shear measurements one expects the described solutions behave like normal Newtonian aqueous solutions. This is in fact the case for small shear rates (Fig. 11.32). In Fig. 11.32 the shear viscosity, which was measured in a capillary viscometer, is plotted vs.the shear rate. One observes a sudden rise of the viscosity at a characteristic shear rate and for y % the solutions show some shear thickening behaviour. Obviously something dramatic has happened to the micelles in the solutions. Some conclusions about what has happened can be drawn from flow birefringence measurements. Some typical results of flow measurements from a Couette system are shown in Fig. 11.33. We note a sudden increase of the flow birefringence at a critical shear rate. For y yc no flow birefringence could be detected. 

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