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The "Weissenberg Effect" or "Rod-Climbing Experiment"In this video clip a dilute (0.025 wt%) solutionof 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).In the experiment a rod is rotated with its end immersed in the fluid outlined above. In the Newtonian case 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 chainsin 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 a) relaxes and b) overcomes the force pushing from below. For a more detailed explanation, and more photographs, see for example p 11-19 D. V. Boger and K. Walters (1993) Rheological Phenomenain Focus, Elsevier Science Publishers, Amsterdam. This effect has important repercussions for polymer solution processing. rod_climbing.avi (1.77 MB) rod_climbing.mov (1.52 MB) rod_climb_highres.jpg (335 KB) |
