Shear thinning/thickening systems

Fumed silica bears the properties of an effective thickener which will not undergo swelling and exhibits chemically inertness. Additionally, thickening by fumed silica results in a non Newtonian system commonly accompanied by a yield point, shear thinning and thixotropy. Thus, fumed silica solves in an excellent manner the requirements of reversible shear thinning under high shear stress but a distinct yield point or high viscosity at lower shear stresses. 

Hard sphere systems are characterized by viscous flow and for low solids loading (less than 5%) they can be described as Newtonian fluid. At higher loadings, cluster formation takes place and the fluid cau acquire shear thinning or thickening behavior. The viscosity and solids loading are correlated with the... 

The above-described thickeners satisfy the criteria for obtaining very high viscosities at low stresses or shear rates. This can be illustrated from plots of shear stress a and viscosity tj versus shear rate y (or shear stress), as shown in Figure 10.25. These systems are described as pseudoplastic or shear thinning. The low shear (residual or zero shear rate) viscosity tj(0) can reach several thousand Pa s, and such high values prevent creaming or sedimentation [24, 25]. 

Simple classifications of fluids can be made on the basis of their rheological profiles. Figure 3.78 shows the (a) shear stress and (b) viscosity profiles for various systems. From Figure 3.78 one may define the following systems. Newtonian systems have a constant viscosity with respect to shear rate. Dilatant (or shear-thickening) systems have a viscosity that increases with respect to shear rate. Pseudo-plastic (or shear-thinning) systems have a viscosity that decreases with respect to shear rate. Yield-stress materials are materials that have an initial structure that requires a finite stress before deformation can occur. The stress that initiates deformation is defined as the yield stress. 

Deviations of liquids from Newtonian behavior are frequently observed for pharmaceutical and biomedical systems. In these, the relationship between stress and the rate of strain is nonlinear, examples of which include pseudoplastic (shear thinning), dilatant (shear thickening), plastic, and Bingham and Ostwald systems (1,17). Such systems are commonly referred to as non-Newtonian systems. 

The degree of concavity is a measure of the shortness, or butterlike quality, of the dispersion. The system is known as shear thinning (34). Naturally, a system displaying opposite concavity would be called shear thickening. Such terms as thixotropic and pseudoplastic are to be avoided, even though they appear frequently in the literature. 

This chapter is an in-depth review on rheology of suspensions. The area covered includes steady shear viscosity, apparent yield stress, viscoelastic behavior, and compression yield stress. The suspensions have been classified by groups hard sphere, soft sphere, monodis-perse, poly disperse, flocculated, and stable systems. The particle shape effects are also discussed. The steady shear rheological behaviors discussed include low- and high-shear limit viscosity, shear thinning, shear thickening, and discontinuity. The steady shear rheology of ternary systems (i.e., oil-water-solid) is also discussed. 

Shear thinning behavior is commonly observed in emulsions. When solids are added to emulsions, both shear thinning and shear thickening behaviors are observed. Figure 32 shows the rheograms of clay-and-oil in water measured by Yan (private communication), where the clay-free oil volume fraction, j30 = 0.2. Part of Figure 32, y > 10 s"1, was also shown by Yan et al. (195). The systems are shear thinning, which is similar to the pure suspension of clay in water and oil in water emulsion... 

Carbopol ETD 2823 is used in this formulation to cost effectively thicken the surfactant system into a gel form - which can be applied to the fabric by brush or sprayed on due to CarbopoVs shear thinning nature. 

In pseudoplastic and dilatant Uquids the viscosity is no longer constant. In the former it decreases and in the latter it increases with increasing shear rate that is to say, the shear stress increases with increasing shear rate less than proportionately in a pseudoplastic and more than proportionately in a dilatant. Pseudoplastics are thus described as shear-thinning and dilatants as shear-thickening fluid systems. These two flow phenotypes can be described by an Equation the power law ... 

Examples of non-Newtonian behavior are shown in Figure 13.3. Shear-thickening (dilatant) is observed when the resistance to deformation increases with the shear rate, whereas the opposite is true for a system undergoing shear-thinning pseudoplastic). In some cases, a minimum critical value of stress, x, is needed for flow to occur Bingham flow). This situation is characteristic of fluids where a structured arrangement of the molecules exists, and therefore a critical x is required to break down the structure. 

An examination of equation (2.14) shows that for any fluid with a finite yield point, the versus Xb curve approaches the Xb axis at zero slope, due to the requirement for such a system that the shear rate must become zero at finite Xb. This may lead to apparent shear-thinning characteristics being ascribed to systems, irrespective of the actual form of their flow curves above the yield point, i.e., whether Bingham plastic, shear-thickening (with a yield stress), or shear-thinning (with a yield stress). 

Here, again, (as in Figure 4-2) system 1 represents a Newtonian liquid (constant viscosity) system 2 is the shear-thinning case (viscosity drops) while system 3 represents the shear-thickening case (viscosity increases). Only when conditions of low shear prevail may one consider the use of the initial constant viscosity i/o, which can be correlated with molecular weight (MW). See Figure 4-4. A typical empirical power law is followed by many polymers. That is. 

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