ABSTRACT
Representative capillary viscometer data. for capillaries having very large L/ D values. plotted as shear stress versus shear rate. assuming no slip. for a I% aqueous Separan AP-30 solution are shown in Fig. 9.12 ]33]: tube diameters range from 0.01906 to 0.1097 em. The curve marked "No Slip" is for coneand-plate viscometer data. and it is clear that the different data sets do not superpose. For a given wall shear stress. decreasing tube diameter leads to progressively higher apparent shear rates due to the presence of wall slip. Fig. 9.13 quantifies the apparent slip phenomenon according to Eq. (9.5). and straight lines result on plotting 32Q/rr.D 3 (the apparent wall shear rate) as a function of I I D while keeping the wall shear stress fixed. The slip velocity calculated from the slope of these straight lines increases with increasing wall shear stress and can be represented as a power-law in the shear stress. Finally. 1-'ig. 9.14 shows the fractional contribution of slip flow to the total flow rate. and this can be substantial for small diameter tubes. especially at low values of the wall shear stress. It is sometimes possible for the tlow rate to be composed almost entirely of slip flow. Ln this situation. the material flows essentially like a plug. The decrease in the
