Concentric cylinder viscometer slip effect

C.G. Qiu and M.A. Rao, Effect of Dispersed Phase on the Slip Coefficient of Apple Sauce in a Concentric Cylinder Viscometer, J. Texture Studies, 16 179-192 (1989). 

Princen and Kiss [13] have studied the rheological properties of emulsions in which the effects of drainage, coalescence and diffusion transfer is much less expressed than in foams. A concentric-cylinder viscometer was used. The slip was estimated by rheograms in the t vs. 0) lx form (t is the stress measured on the inner cylinder wall, co is the angular velocity of the outer cylinder). The Xq values obtained are conform well with those from Eq. (8.24) but Xo (y) function is not a Bingham one, i.e. does not obey Eq. (8.11) at rj = const. 

Qiu, C.-G. and Rao, M. A. 1989. Effect of dispersed phase on the slip coefficient of apple sauce in a concentric cylinder viscometer. J. Texture Stud. 20 57-70. 

The concentric cylinder viscometers are supplied with different inner and outer cylinders such that various gap widths can be formed. For rheological measurements of emulsions and suspensions, care must be taken to ensure a gap width of at least 20 times the suspended particle size in order to avoid wall effects. Moreover, experiments should be conducted with different gap widths to ensure the absence of any wall slip that is usually encountered in emulsion viscosity measurements (J6). However, uniformity of shear rate can be achieved only when the ratio of the gap width to the inner cylinder radius is small. 

Other errors, which could influence the results obtained, are, for example, wall effects ( slipping ), the dissipation of heat, and the increase in temperature due to shear. In a tube, the viscosity of a flowing medium is less near the tube walls compared to the center. This is due to the occurrence of shear stress and wall friction and has to be minimized by the correct choice of the tube diameter. In most cases, an increase in tube diameter reduces the influence of wall slip on the flow rate measured, but for Newtonian materials of low viscosity, a large tube diameter could be the cause of turbulent flow. ° When investigating suspensions with tube viscometers, constrictions can lead to inhomogeneous particle distributions and blockage. Due to the influence of temperature on viscosity (see Section Influence Factors on the Viscosity ), heat dissipated must be removed instantaneously, and temperature increase due to shear must be prevented under all circumstances. This is mainly a constructional problem of rheometers. Technically, the problem is easier to control in tube rheometers than in rotating instruments, in particular, the concentric cylinder viscometers. ... 

Qiu and Rao (Qiu, C. G. and Rao, M. A. J. Texture Stud., submitted) determined slip coefficients and slip velocities for apple sauce in a concentric cylinder viscometer as well as the effect of insoluble solids content on them. Three concentric cylinder units specified in the theory of Mooney (42.) were employed. Rotational speeds were determined with the different concentric cylinder systems at the same magnitude of torque. Figure 2 shows, for one sample of apple sauce, the shear rates uncorrected and corrected for slip plotted against the shear stress. The magnitudes of the flow behavior index of the power law model (Equation 2) did not change significantly due to correction for wall slip however, the magnitudes of the consistency index increased due to wall slip corrections. 

Using their modified concentric-cylinder viscometer -equipped in this case with polished glass inner and outer cylinders to allow unimpeded wall slip, and a mercury pool to eliminate the lower end effect -Princen and Kiss (126) determined the yield stresses, tq, and effective viscosities. 

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