Flow-induced orientational change of aqueous solution of polymer after an abrupt expansion in planar channel flows

Abstract

Flow-induced orientational changes of an aqueous polymer solution in planer channels have been studied utilizing optical rheometric technics. Two kinds of 1:4 abrupt expansion channels with the same width and different heights of 1 mm and 0.2 mm were tested in the experiments. 0.5 wt% xanthan gum solution was examined as the test fluid with non-Newtonian and viscoelastic properties due to the network structures of the polymer. The optical rheometric device measuring birefringence and orientation angle of the polymer was adapted to a microscope. We have obtained remarkable differences in the development of birefringence between these cases. In the case with channel height of 1 mm, the birefringence measured on the centerline of the channel was rapidly decreased just after the expansion, then showed temporal increase. It is considered that this characteristic change in the birefringence corresponded to the rapid disordering and subsequent alignment of the polymer perpendicular to the flow direction because of the negative elongational flow generated after the expansion. After these regions, the polymer gradually aligned parallel to the flow direction. In contrast, the temporal increase in birefringence and alignment to the transverse direction in molecular orientation was not observed in the case with channel height of 0.2 mm. Flow-induced orientational changes of polymers in planar channels with expansions were considerably affected with the combination of local shear rate and elongational rate changes.