STRESS TENSOR MEASUREMENT USING BIREFRINGENCE IN OBLIQUE TRANSMISSION

A method for measuring the stress tensor of liquids obeying the stress -optical rule is presented. In particular, the procedure makes it poss ible to determine the shear stress, and the first and second normal st ress differences for rheometric flows. This technique is an extension of the procedure recently described by Burghardt and coworkers (Brown et al., 1995) wherein light is sent obliquely through a sample sheared between transparent plates. However, in the present development, the light is transmitted in the plane containing the velocity gradient and neutral directions, thereby reducing the necessary optical measuremen ts by one. A polystyrene-tricresyl phosphate (TCP) solution is used as the test sample. The first and second normal stress differences in st eady shear flow measured by this method show good agreement with the m echanical results measured by Madga et al. (1993) using a modified con e and plate rheometer. The transient behavior of the first and second normal stress differences following the start-up of shear flow is also presented.