Xanthan in-situ rheology apparent slip effects

3 Xanthan in-situ rheology apparent slip effects 

In most of the papers on xanthan in-situ rheology discussed in the previous section, results were reported on cases in which purely pseudoplastic behaviour was shown and, in some cases, a lower Newtonian plateau was found. For example, this low-shear Newtonian regime was reported for lower 

Other workers, including Cannella et al (1988) and Willhite and Uhl (1986), mainly present results for higher concentration solutions. 

The mechanism of this apparent slip effect is thought to be molecular surface exclusion, resulting in a layer depleted in polymer at the pore wall (Auvray, 1981 Chauveteau and Zaitoun, 1981 Chauveteau, 1982 Chauveteau et al, 1984). This phenomenon is caused by the entropic exclusion of polymer molecules from the wall of the porous medium and is of particular significance when the dimensions of the macromolecule approach those of typical pore sizes. This effect has been observed in both non-adsorbing (Chauveteau and Zaitoun, 1981 Chauveteau, 1982 Lecourtier and Chauveteau, 1984) and adsorbing (Chauveteau et al, 1984) porous media. 

The IPV model implies that the macromolecules are too large to enter certain parts of the porous medium which are connected by very narrow channels (say 0.5 jum). This may indeed apply for porous materials of very small pore size and hence very low permeability. However, it is more likely that the excluded volume interpretation is the correct model for porous materials of larger pore size (higher permeability) (Chauveteau and Zaitoun, 1981 Chauveteau, 1982 Sorbie et al, 1987d Zaitoun and Kohler, 1987). The effect of surface exclusion or IPV on polymer transport will be discussed in more detail in Chapter 7. 

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