Pseudoplastic Shear Thinning System

In this case, the system does not show a yield value rather, it shows a limiting viscosity ri 6) at low shear rates (that is referred to as residual or zero shear viscosity). The flow curve can be fitted to a power law fluid model (Ostwald de Waele) 

As the power law model [Equation (20.3)] fits the experimental results for many non-Newtonian systems over two or three decades of shear rate, this model is more versatile than the Bingham model, although care should be taken when applying this model outside the range of data used to define it. In addition, the power law fluid model fails at high shear rates, whereby the viscosity must ultimately reach a constant value - that is, the value of n should approach unity. 

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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. 

Pseudoplastic a system where the shear rate increases faster than the applied stress also known as shear-thinning system. 

For most practical suspensions (with c ) > 0.1 and containing thickeners to reduce sedimentation) a plot of a versus y is not linear (i.e. the viscosity depends on the applied shear rate). The most common flow curve is shown in Fig. 3.47 (usually described as a pseudoplastic or shear thinning system). In this case the viscosity decreases with increasing shear rate, reaching a Newtonian value above a critical shear rate. 

M. M. Cross, Rheology of Non-Newtonian Fluids a New Flow Equation for Pseudoplastic Systems, J. Colloids Sci., 20, 417 137 (1965) also M. M. Cross, Relation Between Viscoe-lasiticity and Shear-thinning Behaviour in Liquids, Rheological Acta, 18, 609-614 (1979). 

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]. 

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. 

Pseudoplastic A fluid whose viscosity decreases as the applied shear rate increases. Also termed shear-thinning. Pseudoplastic behavior may occur in the absence of a yield stress and also after the yield stress in a system has been exceeded (i.e., when flow begins). 

Several researchers reported viscoelastic behavior of yeast suspensions. Labuza et al. [9] reported shear-thinning behavior of baker s yeast (S. cerevisiae) in the range of 1 to 100 reciprocal seconds at yeast concentrations above 10.5% (w/w). The power law model was successfully applied. More recently, Mancini and Moresi [10] also measured the rheological properties of baker s yeast using different rheometers in the concentration range of 25 to 200 g dm. While the Haake rotational viscometer confirmed Labuza s results on the pseudoplastic character of yeast suspension, the dynamic stress rheometer revealed definitive Newtonian behavior. This discrepancy was attributed to the lower sensitivity of Haake viscometer in the range of viscosity tested (1.5 to 12 mPa s). Speers et al. [11] used a controlled shear-rate rheometer with a cone-and-plate system to measure viscosity of... 

Effects of Shear Rate and Temperature. In nearly all instances, the polyampholytes showed only slight pseudoplastic or Newtonian behavior in deionized water and salt solutions in the dilute regime. In semidilute solutions near C, some shear thinning was observed by using Contraves rheometry however, the behavior was not pronounced. Only the imbalanced terpolymer systems, which are in essence polyelectrolytes, exhibited pseu-... 

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. 

Such systems are termed pseudoplastic or shear thinning. For such dispersions, as the viscosity decreases, the faster material is pumped through a pipe, sprayed out of a nozzle or mixed with a mixer. As Figure 6.12 shows, pseudoplastic behaviour in dispersions can be caused by alignment, stretching, deformation or... 

There are two rather distinct types of shear-thinning behavior. Systems which show no flow (infinite viscosity) until a critical shear stress Oq, known as the yield stress, is exceeded are said to exhibit plasticity. The viscosities of such systems approach Newtonian limits as shear stress increases. In the other type of behavior, sometimes given the pejorative name pseudoplasticity, Newtonian behavior is approached at both high- and low-shear limits. Recent studiesyield stress, exhibiting Newtonian behavior with extremely high viscosities. Nevertheless, the concepts of plasticity and yield stress can be usefully applied to practical systems, including many adhesives. 

Viscosity may be constant (Newtonian), shear thickening (dilatant), or shear thinning (pseudoplastic) with shear rate. For polymer systems, solution or melt, the viscosity can be related to the molecular weight of the polymer, as discussed in Refs. 4, 8, and 13. 

For a non-Newtonian system, as is the case with most food colloids, the stress-shear rate gives a pseudoplastic curve and the system is shear thinning, i.e. the viscosity decreases with increasing sheeu rate. In most cases the shear stress-shear rate curve can be fitted with the Herschel-Bulkley equation. 

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