Non-Newtonian properties

Second, if we speak about polymer melts, we should take into account their non-Newtonian properties, so that theTi((p) dependence will be different at different rates or shear stresses. This, besides all other things, raises the question on the correct selection of conditions for comparing the viscosity of the systems with different filler content. 

An understanding of non-Newtonian behaviour is important to the chemical engineer from two points of view. Frequently, non-Newtonian properties are desirable in that they can confer desirable properties on the material which are essential if it is to fulfil the purpose for which it is required. The example of paint has already been given. Toothpaste should not flow out of the tube until it is squeezed and should stay in place on the brush until it is applied to the teeth. The texture of foodstuffs is largely attributable to rheology. 

The usual approach for non-Newtonian fluids is to start with known results for Newtonian fluids and modify them to account for the non-Newtonian properties. For example, the definition of the Reynolds number for a power law fluid can be obtained by replacing the viscosity in the Newtonian definition by an appropriate shear rate dependent viscosity function. If the characteristic shear rate for flow over a sphere is taken to be V/d, for example, then the power law viscosity function becomes... 

X correction factor to Stokes law to account for non-Newtonian properties,... 

The foregoing procedure is straightforward if all the particles are of the same diameter (d). However, if the solid particles cover a broad range of sizes, the procedure must be applied for each particle size (diameter dh concentration particle size to the pressure drop APsi, The total solids contribution to the pressure drop is then T,APSj. If the carrier vehicle exhibits non-Newtonian properties with a yield stress, particles for which d < To/(0.2g Ap) (approximately) will not fall at all. 

There have been very few studies of the effects of non-Newtonian properties on flow patterns in hydrocyclones, although Dyakowski et al.,AU have carried out numerical simulations for power-law fluids, and these have been validated by experimental measurements in which velocity profiles were obtained by laser-doppler anemometry. 

An alternative method of reducing the resistance to filtration is to recirculate the slurry and thereby maintain a high velocity of flow parallel to the surface of the filter medium. Typical recirculation rates may be 10-20 times the filtration rate. By this means the cake is prevented from forming during the early stages of filtration. This can be particularly beneficial when the slurry is flocculated and exhibits shear-thinning non-Newtonian properties. This method of operation is discussed by Mackley and Sherman(21) and by Holdich, Cumming and Ismail(22). 

To generate a uniform extrudate geometry at the die lips, the geometry of the manifold must be specified appropriately. Figure 3.15 presents the schematic of a coat-hanger die with a pressure distribution that corresponds to a die that renders a uniform extrudate. It is important to mention that the flow through the manifold and the approach zone depend on the non-Newtonian properties of the polymer extruded. Hence, a die designed for one material does not necessarily work for another. 

In conclusion, we thus find that polymer melts are non-Newtonian in that they have a viscosity that depends on the shear rate y12, or the shear stress ii2 in steady shear flows. This is the most important non-Newtonian property that we encounter in polymer processing. 

The coefficients Ti and b2, like non-Newtonian viscosity, are also found to be shear rate dependent. The non-Newtonian property of exhibiting normal stresses in shear flows plays an important role in processing under situations in which shear stresses vanish, as in extrudate swell, discussed later in this section. 

In addition to the shear thinning effect, other non-Newtonian properties bring about additional complexities in the flow pattern, as demonstrated by Unkriier (37), such as cross-machine flow superimposed on the main machine-direction flow in the inlet region. 

The first centripetal pump resulting from non-Newtonian properties of liquids was suggested by Marcus Reiner in the 1950s. 

Janse I, van Rijssel M, Ottema A, Gottschal JC (1999) Microbial breakdown of Phaeocystis mucopolysaccharides. Limnol Oceanogr 44 1447-1457 Jenkinson IR (1986) Oceanographic implications of non-Newtonian properties found in phytoplankton cultures. Nature 323 435 137... 

All these fluids can show the other non-Newtonian properties that we have already discussed elastic properties, normal stresses, and thickening or thirming of the elongational viscosity. In addition, they can also have a yield stress or a time-dependent viscosity. 

A filled system s rheology depends on conditions of shearing. " Figures 9.25 and 9.26 show the effect of shear stress on two polymers filled with talc." The reaction of both systems differs in the rates of viscosity change and in the character of non-Newtonian properties but the responses of both systems are similar in their reactions to high shear rates where viscosities of the filled and the neat polymer are almost the same. 

W. Muller (Technical University Dortmund) [384] has utilized differently concentrated aqueous solutions of glucose (Newtonian), CMC (pseudoplastic) and PAA (viscoelastic) in experiments in baffled tanks with pitched-blade stirrers and found that in the Re,.fr range of Repff = 10 -10 the nOg values were about a factor of 100 to 10 higher with PAA than with glucose, whereas these values for CMC and glucose only differed by a factor of 5. Only above Rdeff = IO did the non-Newtonian properties cease to have an effect on this relationship (Fig. 3.8). 

Asphalt is a thermoplastic material recovered from distillation bottoms. There is some evidence that it may have been used by Noah to seal the ark. Today, the major uses are in paving for highways and for roofing. Asphalt, depending on its geologic origin, may exhibit Newtonian or non-Newtonian properties. 

Karnis, Goldsmith, and Mason (K5, K5d) performed experiments on the radial migration of rigid spheres suspended in a viscoelastic fluid flowing through a circular tube. Migration towards the axis was observed under conditions where inertial effects would normally be expected to be nil. The observed radial motion is thus apparently attributable to the non-Newtonian properties of the fluid. The unperturbed velocity profile is very flat over the central portion of the tube. Particles placed in this region neither rotated nor moved radially. Experiments are also reported for rods and disks (G9b, K5d). 

The energy of gas dissolved in oil undoubtedly creates the basic force moving the oil through the petroliferous formation. As such, the dissolved gas drive plays an important role in oil recovery. Proper utilization of this drive must therefore always be considered whenever any new EOR methods are applied to reservoirs containing crude with non-Newtonian properties. Methods must be devised to saturate such erodes again with gas within the reservoir, should they be found present there already in the degassed state. 

A change in the reactive medium composition during the chemical process results in the change of the complex of its rheological properties. It is well known [11] that most polymeric materials (solutions and melts) show non-Newtonian properties... 

We think that it is an urgent matter to solve concrete problems taking into accoimt all possible peculiarities different empirical relationships (rheological, kinetic), different geometry, various types of representing boundary conditions, organization of heat and mass transfer, etc. The absence of investigations of the effect of non-Newtonian properties of a liquid on the flow mechanisms of rheokinetic media seems to be a gap in the field of theoretical analysis. 

Finally, for liquid/liquid systems which form stable emulsions and for solid/ liquid systems of high concentrations, it is likely that non-Newtonian properties will be encountered. In such cases Equations 8.10-8.18 should be used. 

As there seems to be no single influence that can be associated with non-Newtonian properties it is not realistic to look for explanations which cover all the effects observed. However, it may be useful to consider behaviour that is related to the individual characteristics of pseudoplasticity and viscoelasticity. Pseudoplastic fluids have a lower viscosity in regions of high shear, as near an impeller, and a higher viscosity in regions of low shear, as distant from an impeller. It is possible that this characteristic leads to mixing behaviour different from that observed for Newtonian fluids, in particular for fluid motion to be more than usually confined to the impeller region. 

However, as laminar mixing is usually associated with fluids of high viscosity, it must also be expected that non-Newtonian fluid properties will be encountered in a significant number of cases. Wilkinson and Cliff report difficulties with the mixing of viscoelastic polyacrylamide in water solutions and Ottino has attempted to calculate the effect of non-Newtonian properties on static mixer performance. Studies have been made of the residence time distributions of Newtonian and non-Newtonian fluids in Kenics mixers -. ... 

The arterial circulation is a multiply branched network of compliant tubes. The geometry of the network is complex, and the vessels exhibit nonlinear viscoelastic behavior. Flow is pulsatile, and the blood flowing through the network is a suspension of red blood cells and other particles in plasma which exhibits complex non-Newtonian properties. Whereas the development of an exact biomechanical description of arterial hemodynamics is a formidable task, surprisingly useful results can be obtained with greatly simplified models. 

A striking feature is the way the so-called diseontinuity in the free rise velocity of a bubble varies with its size. Indeed, Astarita and Appuzzo [1965] noted a six to ten fold increase in the rise veloeity at a eritieal bubble size, as seen in Figure 5.6 for air bubbles in two polymer solutions. Subsequently, similar, though less dramatic results, summarised in a reeent review [DeKee et al., 1996], have been reported. The critical bubble radius appears to hover around 2.6 mm, irrespective of the type or the degree of non-Newtonian properties of the continuous phase, and this value is well predieted by the following... 

Viscosity can alternatively be measured by Brookfield viscometer according to ASTM D 1824 but results will differ because of the differences in shear rates and non-Newtonian properties of plastisols and organosols. 

When minute amounts of polymer cause significant changes in a flow, the key fluid property thought to be associated with the effect is extensional viscosity. Consequently, there is considerable interest in this non-Newtonian property and current research activity is devoted to devising means of measuring the property and to understanding its origins at the microscopic level. To help focus on this property, this survey critically examines the evidence which links extensional viscosity to the observed effects, and reviews the techniques which are used to measure the property. 

Among the materials used as polymer network reagents are linear polyacrylamide (LPA), dimethylpolacrylamide, methylcellulose derivatives, poly(ethylene oxide), and others. The appropriate molecular weight of the polymer is important. Sometimes, blends of different molecular weight are used. For example, the mixture of 2% LPA (MW = 9 mDa) and 0.5% LPA (MW = 50 kDa) is used to separate DNA sequencing reaction products of up to 1000 bases in less than 1 hr, as shown in Fig. 1. The viscosity of this polymer is 30,000 cps. The solution exhibits non-Newtonian properties as the viscosity drops upon the initiation of flow. The use of 2% LPA (MW = 16 mDa) and 0.5% LPA (MW = 250 kDa) at 125 V/cm extends the read length to 1300 bases in 2 hr. ... 

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