A dilute solution (0.025 wt%) of a high molecular weight (2 ×106 g/mol) polystyrene polymer (Polysciences Inc.) is dissolved in a low molecular weight (~100 g/mol), Newtonian, viscous (~30 Pa.s) solvent (Piccolastic, Hercules Inc). Video A shows a rod, with its end immersed in this fluid, being rotated.
In a Newtonian fluid, inertia would dominate and the fluid would move to the edges of the container, away from the rod. Here, however, the elastic forces generated by the rotation of the rod (and the consequent stretching of the polymer chains in solution) result in a positive normal force - the fluid rises up the rod. The bulbous shape remaining at the end of the video is the onset of instability as the mass that has been forced up the rod relaxes and overcomes the force pushing from below.
This rod climbing effect has important repercussions for polymer solution processing. For a more detailed explanation, and more photographs, see for example pp. 11-19 of D. V. Boger and K. Walters (1993), Rheological Phenomena in Focus, Elsevier Science Publishers, Amsterdam.
|A. and B. Non-Newtonian fluids showing the rod climbing effect.|
The full video files can be downloaded at their original location from the old group website.