2020 年 48 巻 2 号 p. 129-135
The flow-induced orientational changes of 0.5-wt% xanthan gum solution in a planar channel with an abrupt contraction followed by an abrupt expansion were examined by measuring the flow-induced birefringence and velocity fields. Four 4:1:4 abrupt-contraction-and-expansion channels with middle slit lengths of 1 mm, 2 mm, 3 mm, and 10 mm were tested in the experiments. How the flow-induced orientation developed near the centerline after the abrupt expansion was found to differ remarkably with the slit length. For slit lengths of 3 mm and 10 mm, the polymer molecules were temporally aligned perpendicular to the flow direction by negative elongational flows generated after the abrupt expansion. By contrast, no similar phenomenon was observed for the slit lengths of 1 mm and 2 mm. These results show that the typical flow-induced orientational changes in a polymer in a planar channel just after an abrupt expansion are affected substantially by the length of the middle slit.