Couette flow birefringence

Flow birefringence of polymer solutions is, in general, measured with the aid of an apparatus of the Couette type, containing two coaxial cylinders. One of these cylinders is rotated at constant speed, the other is kept in a fixed position. The light beam for the birefringence measurement is directed through the annular gap between these cylinders, in a direction parallel with the axis of the apparatus. In this way, the difference of principal refractive indices An is measured just in the above defined plane of flow (1—2 plane). 

J.S. Lee and G.G. Fuller, The spatial development of transient couette flow and shear wave propagation in polymeric liquids by flow birefringence, J. of Non Newt. Fluid Mech., 26, 57 (1987). 

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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