Concentration dependent changes of apparent slip in polymer solution flow

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Abstract
In order to prove the depletion hypothesis of ‘‘apparent slip,’’ velocity profiles in ducted flow of aqueous polyacrylamide (PAM) solutions were measured to a distance of d=0.15 μm from the wall and the dependency of the slip velocity on the PAM concentration c∞ was determined. As the slip velocity exhibited exceptionally high values when using glass surfaces all quantitative results were obtained with rectangular glass ducts. For c∞ <0.05% (wt. %), the assumed slip layer lining the wall was identified by an experimental determination of the velocity profile within this layer, as well as of its width δ. Intersection of this profile with the wall position at zero velocity verified the slip as being ‘‘apparent.’’ The width δ decreased with increasing c∞ from δ=0.3 μm at c∞=0.005% to δ≤0.15 μm at c∞ ≥0.05%; 0.15 μm represents the spatial resolution of the anemometer used. The ratio of the slip velocity to wall shear stress, i.e., the slip coefficient—which was constant for each concentration—decreased with i...

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