Adsorption-entanglement layers in flowing high-molecular weight polymer solutions

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This four-part series is concerned with thick multimolecular layers, termed “adsorption-entanglement layers”, which according to our observations develop during flow of high molecular weight polymer solutions along the solid surfaces defining the flow channels. It is concerned with the formation and nature of these layers and with the effect they have on fluid transport in general and viscosity measurements in particular. In the present first part the existence of such layers is established through a flow visualization method and the layer formation diagnosed in situ is correlated with macroscopic flow effects. Two flow geometries are used, Couette and channel flow. In the Couette flow, where the shear rate is kept constant, the adsorption-entanglement layers first form, then thicken and eventually partially break off until a steady layer thickness is reached. These effects are reflected by a concommitant increase and subsequent decrease and final levelling off of the apparent viscosity as a function of shearing time, there being a near quantitative correlation between the measured changes in apparent viscosity and layer thickness. In channel flow, where it is the pressure head which is kept constant, the formation and thickening of layers leads to a decrease in flow rate with flow time, amongst others giving the impression of shear thickening. It is demonstrated that owing to the fact that adsorption entanglement layers alter the geometry of the flow channel the time dependence of flow effects, per se need not indicate shear thickening (or thinning), in fact that there is any departure from Newtonian flow behaviour. The above is of obvious consequence for the rheological interpretation of flow behaviour of solutions of very high molecular weight polymers.