Relative viscosity, effect concentration

For scaly fillers the increase of relative viscosity with filler concentration is not as pronounced as in case of fibrous fillers [177,178]. Owing to filler orientation, the flow curves for systems with different concentrations of a fibrous and a scaly filler may merge together at high shear rates [181]. In composites with a dispersed filler the decrease of the effective viscosity of the melt with increasing strain rate is much weaker. 

Not all suspensions will exhibit wall slip. Concentrated suspensions of finely ground coal in water have been found to exhibit wall slip [Fitzgerald (1990)]. This is to be expected because the coal suspension has a much higher apparent viscosity than the water. In contrast, when the liquid is a very viscous gum, the addition of solids may have a relatively small effect. In this case, the layer at the wall will behave only marginally differently from the material in the bulk. 

Figure 4.8 Effect of increasing the powder concentration in a slurry on relative viscosity. From J. S. Reed, Principles of Ceramics Processing, 2nd ed. Copyright 1995 by John WUey Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Figure 4.21 Effect of axial ratio and particle concentration on relative viscosity. Data are for tobacco mosaic virus particles. Adapted from M. A. Lauffer, 7. Am. Chem. Soc., 66, 1188. Copyright 1944 by The American Chemical Society, Inc.

Variations in the temperature coefficient of viscosity with solvent, which have also been presented as evidence of association in concentrated solutions (135,143), could be similarly related to differences in Ta among the solutions. When free draining behavior is a possibility, the relative viscosities in different solvents should be compared at the same value of Co f°r the mixtures (that is, at constant free volume rather than at constant temperature). In any case, it is clear that a very well planned series of experiments is necessary in order to test for the existence of additional specific effects such as association. These comments are not meant to suggest that association can not occur at moderate concentrations. Indeed, the existence of association in various forms of polymethyl methacrylate seems well established (144). The purpose is rather to advocate that less specific causes be eliminated before association is inferred from viscosity measurements alone. 

Figure 1. Effects of the concentration of dextrans with various molecular weights on three indices of BBC aggregation (16). MAI indicates the average number of RBCs in each aggregation unit counted under the microscope. ESR is the maximum rate of sedimentation of erythrocytes in a calibrated tube, with corrections made for changes in viscosity and density of the suspending medium following the addition of dextrans. The relative viscosity (t)r) is the ratio of the viscosity of RBC suspension to that of the suspending medium at a shear rate fo 0.1 sec 1. The RBC concentration of the suspension was 1% for MAI and 45% for ESR and r)r measurements. The vertical bars represent SEM. (A), Dx 40 (O), Dx 80 (M), Dx 150 (A), Dx 500 ( ), Dx 2000.

In the concentration range regarding the ED processes, the effective diffusion coefficient (Z>B) can be predicted via the Gordon relationship (Reid et al, 1987), which accounts for the partial derivative of the natural logarithm of the mean molal activity coefficient (y+) with respect to molality (m) and solvent relative viscosity (rjr) ... 

Organic photochemical reactions conducted in micellar solutions are reviewed from the standpoint of systematizing and correlating published results. Five common effects are found to distinguish and characterize micellar photochemistry relative to conventional solution photochemistry super cage effects, local concentration effects, viscosity effects, polarity effects, and electrostatic effects. These effects can contribute to the occurence of enhanced selectivity and efficiency of photoreactions relative to those in conventional homogeneous solution. 

The relative viscosity (rjy) changes for benzene solutions of HO (0.305M) at 293 K, with added A A and BA isolated from FB57-42, are shown in Figure 2 (Curves 2 and 5). The results indicate that at a given concentration (above 0.035M) and temperature, BA has a larger effect on viscosity than does AA. 

Yoo, B. and Rao, M. A. 1995. Yield stress and relative viscosity of tomato concentrates effect of total solids and finisher screen size. J. Food Sci. 60 777-779,785. 

Figure 11 shows the relative-viscosity-concentration behavior for a variety of hard-sphere suspensions of uniform-size glass beads. Even though the particle size was varied substantially (0.1 to 440 xm), the relative viscosity is independent of the particle size. However, when the particle diameter was small ( 1 fJLm), the relative viscosity was calculated at high shear rates, so that the effect of Brownian motion was negligible. Figure 8 shows that becomes independent of the particle size at high shear stress (or shear rate). 

Effects of Solids Size. The effect of solids size on the viscosity of the emulsion-solids mixtures is shown in Figure 19 for synthetic OAV emulsions. The oil concentration (solids-free basis) is 60% by volume, and the solids used are silica sand. The comparison is made at shear stresses of 6 and 14 Pa. The viscosity is expressed as the relative viscosity (t7ows/ 7ow)t lhat is, the viscosity of the emulsion-solids mixture divided by the viscosity of the solids-free emulsion. At low solids volume fraction (<0.1), solids size has little effect. 

These effects can be illustrated by estimating values of < from the Sherman model [3 ] from values of relative viscosity and average particle diameter. In Table II we can see the influence of NaCl concentration on a for the two emulsions shown in Figure 6. At the NaCl concentrations of minimum viscosity, values of a also show minima. 

The effect of osmotic pressure in macromolecular ultraflltra-tlon has not been analyzed in detail although many similarities between this process and reverse osmosis may be drawn. An excellent review of reverse osmosis research has been given by Gill et al. (1971). It is generally found, however, that the simple linear osmotic pressure-concentration relationship used in reverse osmosis studies cannot be applied to ultrafiltration where the concentration dependency of macromolecular solutions is more complex. It is also reasonable to assume that variable viscosity effects may be more pronounced In macromolecular ultra-filtration as opposed to reverse osmosis. Similarly, because of the relatively low diffuslvlty of macromolecules conqiared to typical reverse osmosis solutes (by a factor of 100), concentration polarization effects are more severe in ultrafiltration. 

Substitution of a typical value for HAF black (A = 105 cc/100 g) leads to ceff/c = 2.2, which is also of the order required to reconcile Smit s relative viscosity data. (Eq. 17 comes reasonably close to predicting the effective filler concentration for the quasi-equilibrium stress data of Fig. 11, giving rise to shift factors a — 0.62, 0.76, 1.00 and 1.10 see Section V-3.) The possibility of a significant occluded polymer contribution was considered by Smit, but dismissed on the basis of what appears to be an overestimate of the packing density of the particles in the primary structure aggregates. 

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