Macromolecules in Nonhomogeneous Velocity Gradient Fields: Rheological and Diffusion Phenomena
Kinematically nonhomogeneous* flows are common in rheological measurement situations and in flows of technological import for polymers. Rheological anomalies1,2 and migration phenomena3–5 have been observed and/or speculated upon in these flows. Apparent “slip” at liquid-solid interfaces (for example, viscosity decreasing with channel size in small capillaries) has been reported in some such flows of macromolecular liquids. Much discussion has centered on the molecular origins of the observations leading to these reports.6,7 At least three identifiable alternatives are possible. 1. When large molecules are present in the liquid, they sample a larger portion of the flow. Thus, they “detect” in some sense the nonuniform velocity gradient field and this in turn affects the contribution they make to the fluid stress. 2. Macro-molecules migrate away from the walls leaving a concentration-depleted layer which produces lower fluid viscosity in those regions and apparent “slip”.
KeywordsHomogeneous Flow Undisturbed Flow Plane Poiseuille Flow Rouse Model Lower Fluid Viscosity
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