Flow behavior shear-thickening

Rheology is the science of deformation and flow behavior of materials under the influence of parameters such as shear rate and time. Liquids with viscosity not dependent on shear rate are termed Newtonian liquids while non-Newtonian liquids exhibit changes in viscosity depending on shear rate. Most paints exhibit non-Newtonian flow, which is desirable. Important types of non-Newtonian flows are pseudoplastic flow behavior (shear thinning), thixotropic flow behavior (time-dependent shear thinning,) and dilatant flow behavior (shear thickening). Pseudoplastic or thixotropic flow is often desirable in coatings because ... 

A paste that exhibits opposite behavior—shear thickening—is called dilatant. Consider Silly Putty. Under low shear the material rolls and flows easily but if pulled quickly, breaks. Liquids that show no change in viscosity with shear rate, such as water, are called Newtonian (Fig. 4.8). 

For a Hquid under shear the rate of deformation or shear rate is a function of the shearing stress. The original exposition of this relationship is Newton s law, which states that the ratio of the stress to the shear rate is a constant, ie, the viscosity. Under Newton s law, viscosity is independent of shear rate. This is tme for ideal or Newtonian Hquids, but the viscosities of many Hquids, particularly a number of those of interest to industry, are not independent of shear rate. These non-Newtonian Hquids may be classified according to their viscosity behavior as a function of shear rate. Many exhibit shear thinning, whereas others give shear thickening. Some Hquids at rest appear to behave like soHds until the shear stress exceeds a certain value, called the yield stress, after which they flow readily. 

Certain polymeric systems can become more viscous on shearing ( shear thickening ) due to shear-introduced organization. These systems become more resistant to flow as the crystals form so that the introduction of the shear increases their viscosity. Figure 6.5 shows the viscosity versus strain rate relationship for Newtonian and non-Newtonian fluids, highlighting the differences in their behaviors. 

Oscillatory shear experiments are the preferred method to study the rheological behavior due to particle interactions because they directly probe these interactions without the influence of the external flow field as encountered in steady shear experiments. However, phenomena that arise due to the external flow, such as shear thickening, can only be investigated in steady shear experiments. Additionally, the analysis is complicated by the different response of the material to shear and extensional flow. For example, very strong deviations from Trouton s ratio (extensional viscosity is three times the shear viscosity) were found for suspensions [113]. 

Figure 1-2 Basic Shear Diagram of Shear Rate versus Shear Stress for Classification of Time-Independent Fiow Behavior of Fiuid Foods Newtonian, Shear-Thinning, and Shear-Thickening. Also, some foods have yield stress that must be exceeded for flow to occur Bingham and Herschel-Bulkley (H-B).

Resolution of the velocity data and removal of data points near the center of the tube which are distorted by noise aid robustness of the curve fit the polynomial curve fit introduced a systematic error when plug-like flow existed at radial positions smaller than 4 mm in a tube of 22 mm diameter. The curve fit method correctly fit the velocity data of Newtonian and shear-thinning behaviors but was unable to produce accurate results for shear-thickening fluids (Arola et ah, 1999). 

Figure 4-22 The Flow Curves of Cross-Linked Waxy Maize Samples Heated at 120°C for 5, 15, and 30 min were More Viscous After Shearing, that is. They Exhibited Time-Dependent Shear-Thickening (antithixotropie) Behavior.

For the 4% tapioca starch dispersion heated at 70°C for 5 min, values of r]a decreased with y, but those of rj increased with co after an initial decrease (Figure 4-32). For the data shown in Figure 4-32, the critical frequency (a>c) at which the transition occurred was about 100 rad s . In comparison to yc values for shear-thinning to shear-thickening flow behavior of 2.6% tapioca and cowpea starch (Okechukwu and Rao, 1996) dispersions, the value of cuc = 100 rad s is lower. It is possible that the rja versus y data of the 4% dispersion heated at 70°C for 5 min... 

Kumar, P., C.L. Martin, and S. Brown, Shear Rate Thickening Flow Behavior of Semisolid Slurries, Metallurgical Trans. A 24(A) 1107-1116 (1993). 

Aqueous pectin dispersions show flow behavior similar to many other polysaccharide solutions. Flow curves of specific viscosity rpp vs. shear rate have a Newtonian plateau (constant r sp) at low shear rates, followed by a shear thinning region at moderate shear rates (Morris et al., 1981). Most pectin solutions have relatively low viscosity compared to some other commercial polysaccharides, such as guar gum, mainly because of the lower MW. Consequently, pectin has limited use as a thickener. 

When describing dilatant behavior, the maximum stretch rate, e, in the converging flow at the contraction is a better parameter, but more difficult to be calculated. Instead of the term stretch rate, other authors also used deformation rate (e.g., Chauveteau, 1981) or elongational rate (e.g.. Sorbic, 1991). The shear-thickening viscosity is also called elongational viscosity (often referred to as the Trouton viscosity Sorbie, 1991) or extensional viscosity in the literature. James and McLaren (1975) reported that for a solution of polyethylene oxide (a flexible coil, water-soluble polymer physically similar to HPAM), the onset of elastic behavior at maximum stretch rates was of the order of 100 s and shear rates of the order of 1000 s. In this instance, the stretch rate is about 10 times lower than the shear rate. However, some authors use shear rate instead of stretch rate in defining the Deborah number—for example, Delshad et al. (2008). 

In Equation (2), n is the flow behavior index (-),K is the consistency index (Pa secn), and the other terms have been defined before. For shear-thinning fluids, the magnitude of nshear-thickening fluids n>l, and for Newtonian fluids n=l. For PFDs that exhibit yield stresses, models that contain either (Jo or a term related to it have been defined. These models include, the Bingham Plastic model (Equation 3), the Herschel-Bulkley model (Equation 4), the Casson model (Equation 5), and the Mizrahi-Berk model (Equation 6). 

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