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 viscosities of most real shear-thinning fluids approach constant values both at very low shear rates and at very high shear rates that is, they tend to show Newtonian properties at the extremes of shear rates. The limiting viscosity at low shear rates mq is referred to as the lower-Newtonian (or zero-shear /x0) viscosity (see lines AB in Figures 3.28 and 3.29), and that at high shear rates Mo0 is the upper-Newtonian (or infinite-shear) viscosity (see lines EF in Figures 3.28 and 3.29). 

Equation 5.2 is found to hold well for non-Newtonian shear-thinning suspensions as well, provided that the liquid flow is turbulent. However, for laminar flow of the liquid, equation 5.2 considerably overpredicts the liquid hold-up e/,. The extent of overprediction increases as the degree of shear-thinning increases and as the liquid Reynolds number becomes progressively less. A modified parameter X has therefore been defined 16 171 for a power-law fluid (Chapter 3) in such a way that it reduces to X both at the superficial velocity uL equal to the transitional velocity (m )f from streamline to turbulent flow and when the liquid exhibits Newtonian properties. The parameter X is defined by the relation... 

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. 

A popular approach to improve ocular drag bioavailability is to incorporate soluble polymers into an aqueous solution to extend the drug residence time in the cul-de-sac. It is reasoned that the solution viscosity would be increased and hence solution drainage would be reduced. The more commonly used viscolyzing agents include PVA and derivatives of cellulose. Cellulosic polymers, such as methylcellulose, hydroxyethylcellulose (HEC), hydroxypropyl-methylcellulose (HPMC) and hydroxypropylcellulose (HPC), are widely used as viscolyzers showing Newtonian properties. They have common properties ... 

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. 

It should be noted that these rules assume that the liquid flowing over the system has Newtonian properties and is fully synchronized with the disk rotation. The influence of gravity is also neglected and it is assumed that the surface is completely wetted. 

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... 

---