Rheo‐optics of liquid‐crystalline polymers in complex geometries
Résumé
The flow of isotropic and liquid-crystalline hydroxypropylcellulose-water solutions is studied in a transparent slit die with several geometries including divergent and convergent configurations. The flow is observed between crossed polars. In addition to the two different flow regimes that are known in shear (zones I and II), a new flow behavior is observed (zone III). Zone I is characterized by a large macroscopic orientation, as seen between crossed polars. It is localized where either shear rates or elongation rates are large. The extinction angle is a function of the position in the die. It is similar to the flow-aligning regime observed at high shear rates. Zone II is much less oriented, due probably to a large density of defects. It is typical of the low shear rate region observed in simple shear. Zone III exhibits a peculiar behavior with long-range orientation instabilities. It is always localized after a diverging zone. Flow around obstacles and colliding streams are also studied.