ABSTRACT

FIGURE 9.13 Shear stress versus 1/D for the data of Fig. 9.12. (From Ref. 33.)

That the Mooney analysis yields reasonable and accurate results can be confirmed by flow visualization experiments. This is demonstrated in Fig. 9.1 I where it is found that the slip velocity of concentrated suspensions calculated according to Eq. is the same as that measured using a marker line method. Recently nuclear magnetic resonance (NMR) imaging techniques have become available for the experimental determination of the velocity profile of polymer solutions very close to a solid surface. Rofe et al. 1341 have made such measurements on aqueous xanthan gum solutions and shown that NMR imaging gives results that are identical to those obtained with the help of Eq. (9.5). Before leaving the topic of apparent wall slip of polymer solutions, we want to reassure readers that this phenomenon docs not influence viscometric data obtained on polymer solutions in the usual manner. ln particular, there is no slip in a cone-

and-plate viscometer and in a small-gap Couette viscometer because wall slip occurs only in situations where the shear rate varies with position within the flow field; in these situations. it is postulated that macromolecules move from regions of high shear stress to regions of low shear stress [35a]o Even when a capillary viscometer is used, wall slip is unlikely to be a problem because the shear rates are usually very high, and the contribution of slip flow is likely to be quite small (see Figo 9 0 14 ). However, capillaries with large Ll D values may favor slip [35b,c].