Yield-pseudoplasticity

Pseudoplastic with yield Pseudoplastic or Newtonian Toothpaste, lipstick, grease, oil-well... 

Yield-dilatant (n > 1) materials are rare but several cases of yield-pseudoplastics exist. For instance, data from the literature of a 20% clay in water suspension are represented by the numbers To = 7.3dyn/cm, K = 1.296dyn(sec)"/cm and n = 0.483 (Govier and Aziz, 1972, p. 40). Solutions of 0.5-5.0% carboxypolymethy-lene also exhibit this kind of behavior, but at lower concentrations the yield stress is zero. 

In most practical cases, oil field cement slurries contain several water-soluble organic additives. Therefore, cement interstitial liquid is an aqueous solution that is likely not to behave as a Newtonian fluid. Specifically, if the organic additives are long-chain polymers, the interstitial fluid will display a pseudoplastic behavior, as described, for instance, by the power law model. In turn, the slurry will display a yield pseudoplastic behavior as described for example by the Herschel-Bulkley model (see previous sections). 

Block polymers, owing to the tendency for formation of regular strucmres tailored by molecular design, are ideal models for compatibilized, two-phase polymer blends or alloys. Blends do show similar rheological behavior, e.g., yield, pseudoplasticity, thixotropy, structural rearrangements, but since the morphology is more difficult to control, the interpretation of data could present serious difficulties. 

A fluid with a linear flow curve for Ty > ro is called a Bingham plastic fluid and is characterised by a constant plastic viscosity (the slope of the shear stress versus shear rate curve) and a yield stress. On the other hand, a substance possessing a yield stress as well as a non-linear flow curve on linear coordinates (for Xyx > ro ), is called a yield-pseudoplastic material. Figure 1.8 illustrates viscoplastic behaviour as observed in a meat extract and in a polymer solution. 

Figure 1.8 Representative shear stress-shear rate data showing viscoplastic behaviour in a meat extract (Bingham Plastic) and in an aqueous carbopol polymer solution (yield-pseudoplastic)...

The combination of thixotropy and pseudoplasticity in a tape casting slip generally adds up to yield-pseudoplasticity. Yield-pseudo-... 

Hanks, R, W., The axial laminar flow of yield-pseudoplastic fluids in a concentric annulus, Ind. Eng. Chem. Process Des. Dev. 75(3) 488-493 (1979). 

Bingham plastic Dilatant Pseudoplastic Yield pseudoplastic... 

Certain slurries require a minimum level of stress before they can flow. An example is fresh concrete that does not flow unless the angle of the chute exceeds a certain minimum. Such a mixture is said to posses a yield stress magnitude that must be exceeded before that flow can commence. A number of flows such as Bingham plastics, pseudoplastics, yield pseudoplastics, and dilatant are classified as time independent non Newtonian flu ids. The relationship of wall shear stress versus shear rate is of the type shown in Figure 3 9 (a), and the relationship between the apparent viscosity and the shear rate is shown in Figtne 3-9 (b). The apparent viscosity is defined as... 

Yield pseudoplastic fluids are time independent non-Newtonian fluids and are characterized hy the following ... 

Equation 3-5 i is known as the Herschel-Buckley equation of yield pseudoplastics and is accepted by most slurry experts to describe the rheology of yield pseudoplastics with... 

The behavior of yield pseudoplastics can be expressed by the Carson model as described by Lapasin et al. (1998) ... 

Yield stress for a Bingham plastic or yield pseudoplastic... 

Yield pseudoplastics were described extensiveiy in Chapter 3. Exampies were listed in Table 3.11. 

For yield-pseudoplastics, Hanks and Ricks (1978), Heywood (1991) proposed to define a modified Hedstrom Number as... 

FIGURE 5-5 Values of the critical value of the Hanks and Ricks Reynolds number versus the flow index n for yield pseudoplastic slurries. 

The Wilson-Thomas Method was developed in the 1980s for yield pseudoplastic and power law slurries. Wilson (1985) and Thomas and Wilson (1987) assumed that the fluid... 

Torrance (1963) developed a theoretical equation for yield pseudoplastics in fully turbulent flow (at high Reynolds numbers) in rough pipes as... 

For a yield pseudoplastic, Slatter et al. (1996) defined their Reynolds number, Re in terms of the friction velocity, consistency factor K, and power coefficient n, as weU as roughness d ... 

Laminar component of fanning friction factor for a yield pseudoplastic Eanning friction factor for a pseudoplastic in a laminar regime Tomita laminar friction factor Turbulent component of farming friction factor... 

A behaviour of the dense fine grained slurries in laminar flow regime can be described by Bingham fluid model or the yield pseudoplastic rheological model, respectively... 

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