Shear-thinning character

Figure 12.29 shows that the die does not end at the plane z = 0. Because polymer melts are viscoelastic fluids, it extends to downstream to the end of the die lip region so that a uniform recent flow history can be applied on all fluid elements. In deriving the die design equation, we disregarded the viscoelasticity of the melts, taking into account only their shear thinning character. 

Increasing the levels of Cab-O-Si1 and/or pigment in the print paste does not alter the plastic viscosity, but it increases the Bingham yield stress. The paste thus has a greater "shear thinning" character at higher Cab-O-Sil or pigment levels. 

Here, X is still the relaxation time of the fluid, but now we see that it is also responsible for the shear thinning character of the viscosity. It is a consequence of the finite time required for the polymer molecules to recover to their equilibrium configuration relative to the rate at which they are deformed. Because of the general similarity between (11) and (13) a quantitative expression for X can be obtained by equating these expressions and solving for X ... 

There is also a friction factor chart drawn by Dodge and Metzner plotting the Fanning friction factor versus the Metzner-Reed Reynolds number (although this has been subsequently redrawn for convenience with (j), see ref. 4). This chart shows that the friction factor decreases with n in turbulent flow—that is, it decreases as shear-thinning character increases. If you are studying this chart, be aware that the only experimentally verified data for turbulent flow is that given for m = 1.0 to n = 0.4. 

A particular conclusion from this theoretical analysis is that, if a crack has faces that are separated by a thin layer of fluid, so that normal components of traction and displacement are transmitted across the crack but the faces are free with regard to shear components of traction and displacement, then there will be a scattered wave however thin the fluid layer is. This is perhaps not surprising. A Rayleigh wave can exist only because solids can support both longitudinal and shear waves, and the greater part of the displacement in a Rayleigh wave is shear in character ( 6.3). Of course, liquids can support shear stress over a short distance. In a liquid of viscosity r/, and density po, at a frequency o) the amplitude of a shear wave decays by a factor e over a distance... 

Cubic liquid crystalline systems have been described as clear, stiff gelsJ As such, they show shear thinning after an apparent yield stress has been exceeded. The viscoelastic properties are also typical for the gel character a broad linear viscoelastic range and a frequency-independent elastic component, which is considerably higher than the viscous component, are observed. ... 

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... 

It was noted earlier in this chapter that the flow of both polyacrylamide and xanthan solutions in a capillary is similar in character, i.e. it is shear thinning... 

The viscosity increases as the polymers grow, because more work must be done to produce flow of the sol. The liquid within the clusters is not available for flow, and this increases the effective volume fraction of polymer. When the clusters become large (later than 1176 h in Fig. 3) they can be broken by the rotating cylinder, so the sol becomes shear thinning, which means that the viscosity decreases with increasing shear rate. When the network begins to form, the sol can support a static load without flowing this is elastic behavior, and it can be seen in the data collected later than 1327 h in Fig. 3a. The sol behaves elastically until the stress reaches a certain level (the yield stress) after which it flows the yield stress is 25 Pa at 1339 h and 75 Pa at 1356 h in Fig. 3a. The hysteresis seen in the curves for 1339 and 1356 hours In Fig. 3b indicates that the network is broken by the shear stress and does not recover immediately as Tj is reduced below the yield stress. As the gel point approaches, the elastic character of the network increases... 

The yield stress of a foam depends to a considerable extent on the character of foam interaction with the tube walls or the cylindrical surface of the viscometer, used in the study of its rheological properties. At low flow rates and smooth tube walls the maximum shear stress of the foam layers contacting the wall can be less than the shear stress of the foam matrix (shear of bubble layers). Hence, the foam flow will occur as a movement of a continuous medium in a cylinder covered with a thin lubricating layer of thickness 2-10 pm [9,16], In this case t0 is ca. 1 Pa, that is, much less than its theoretical value. 

Nucleic acids are water soluble because of the polyanionic character of the molecule. It is possible to obtain viscous aqueous solutions of DNA up to 1 % (w/ v). The long, thin DNA structure means that the molecule is very susceptible to cleavage by shearing or sonication in solution, which results in a reduction in solution viscosity. The nucleic acids are precipitated from aqueous solution by the addition of alcohol. 

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