Viscosity electrostatically stabilized suspensions

In Brownian suspensions, as (p increases, the slope of the viscosity-shear rate curve in the shear-thickening regime typically increases, and for electrostatically stabilized suspensions at high-volume fractions, it can even become a discontinuous jump. At shear rates above the shear-thickening regime, there is typically a second shear-thinning regime (see... 

Ackerson and Clark (158), Chen and Zukoski (159), Stevens et al. (160), and Chen et al. (161) observed a different kind of discontinuous viscosity response to an order-disorder transition for electrostatically stabilized suspensions at a relatively low solid concentrations, where the viscosity displays a discontinuous drop at certain shear rate. Apart... 

Figure 4.41 shows the effect of the adsorbed PVA on the relative viscosity of the suspension for suspensions prepared at a pH of 3.7. The data also show the viscosity for an electrostatically stabilized suspension prepared at a pH of 7.0 without any PVA. For the suspensions containing no PVA, we see that the stabilized suspension (pH = 7.0) has a fairly low viscosity and shows Newtonian behavior, while the unstabilized suspension (pH = 3.7) has a much higher viscosity and shows a high degree of shear thinning. 

Direct Coagulation Casting. In this NEAR NET SHAPE technique for forming complex shapes, an aqueous electrostatically stabilized suspension of low viscosity is cast into a non-porous mould and then coagulated by changing the pH or by a delayed reaction catalyzed by enzymes, to form a stiff, wet green body. 

Figure 4.7(a) shows the relative viscosity versus shear rate behavior for suspensions at pH = 3.7 with varying polymer adsorption in the range of 0 to 1.1 mg adsorbed PVA/g silica. For the sake of comparison, the relative viscosity versus shear rate plot is also shown in the same figure for an electrostatically stabilized suspension prepared at pH = 7.0 with no PVA. At pH = 7.0, wherein the zeta potential is approximately -55 mV, the silica suspension is extremely well... 

Figure 4.7 Variation of relative viscosity with shear rate for 20 vol% silica suspensions prepared at pH = 3.7 with varying concentrations of poly(vinyl alcohol). The relative viscosity vs. shear rate plot for an electrostatically stabilized suspension (pH = 7.0) with no poly(vinyl alcohol) is also shown. (Reprinted from Ref. 19 with kind permission from The American Ceramic Society Inc., Westerville, Ohio, USA.)...

This section on concentrated suspensions discusses the rheological behavior of sj tems which are colloidally stable and colloidally unstable suspensions. For stable sj tems, the rheology of sterically stabilized and electrostatically stabilized systems wiU be considered. For sterically stabilized suspensions, a hard sphere (or hard particle) model has been successfid. Concentrated suspensions in some cases behave rheologically like concentrated polymer solutions. For this reason, a discussion of the viscosity of concentrated polymer solutions is discussed next before a discussion of concentrated ceramic suspensions. 

Figure 4.11 shows the plots of shear stress versus shear rate for toe suspensions prepared with alumina powder and different surface modifying agents. The suspensicm wito zircoaluminate is well dispersed as indicated by the Newtonian flow behavior and low suspension viscosity which was determined to be about 2.8 centipoise. The good dispersion is also indicated from a comparison with the electrostatically stabilized alumina suspension prepared at pH = 4.0 with no chemic additives as shown in Figure 4.12. 

In some suspensions, a particle size distribution that is heavily weighted toward the smaller sizes will represent the most stable suspension. In such cases changes in the size distribution curve with time yield a measure of the stability of the suspensions. The particle size distribution also has an important influence on the viscosity. For electrostatically or sterically interacting particles, suspension viscosity will be higher, for a given mass concentration, when particles are smaller. The viscosity will also tend to be higher when the particle sizes are relatively homogeneous, that is, when the particle size distribution is narrow rather than wide. 

Suspensions can show varying rheological, or viscosity, behaviors. Sometimes these properties are due to stabilizing agents in the suspension. However, typically particle-particle interactions are sufficient to cause the suspension viscosity to increase because of electrostatic interactions or simply particle crowding. ... 

Figure 4.9(a) and (b) shows the relative viscosity versus shear rate behavior in these two solvents for varying PVB concentration from 0 to 2.0vol%. In either of the solvents, highly shear-thirming behavior is observed when no PVB is added. These suspensions show poor stability against flocculation due to relatively low zeta potentials (< 25 mV) and therefore, low electrostatic repulsive forces [152]. 

An inspection of Table 2, where the deflocculant ability of Aluminon is compared with that of other dispersants investigated by the Authors in previous papers, permits to outline a scale of effectiveness based on the values of optimum dosage and minimum apparent viscosity. In the light of the results reported in Table 2 it can be stated that sodium polyacrylate (Reotan L) and sodium polyphosphate behave as the best deflocculants for kaolin suspensions owing to their dispersing mechanism, acting by both electrostatic repulsion and steric stabilization. Aluminon presents an intermediate capacity of dispersion stabilization. 

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