Rheo-optics of hydroxypropylcellulose solutions in Poiseuille flow

The behavior of an aqueous solution of hydroxypropylcellulose in the liquid crystalline phase is investigated when it is flowing in a rectangular channel. Rheological characterization shows that the viscosity vs. shear rate curve follows the typical "three region" pattern, with the intermediate plateau of region II extending over a relative large range of shear rates (more than one decade). Two complementary rheo-optical determinations are performed. Velocity profiles across the channel thickness are measured by a hydrogen bubble visualization technique. Texture evolution is monitored by in situ optical microscopy. Accurate focusing inside the sample thickness allows observation in real time of the texture at various shear rates, as generated in the Poiseuille type of flow in the channel. It is shown that the velocity profiles can be accurately predicted by assuming that the flow in the channel is purely viscous, and using only the viscosity data described above. It is also shown that the morphology of the texture generated inside the flowing system is a function of the local shear rate. In particular, an elongated structure is observed when the shear rate exceeds the critical value corresponding to the onset of region II in the viscosity curve.