Viscoelasticity shear thinning flow

In order to illustrate the specific material properties of polymers, we compare a viscous fluid (silicone oil) with a viscoelastic shear thinning fluid (aqueous polyethylene oxide solution). These fluids are used as model fluids in order to show the flow behavior limits for polymer melts, which corresponds to the behavior of a viscous fluid at very low shear rates and to the behavior of a shear thinning fluid at very high shear rates. 

Flow of hexagonal liquid crystalline systems is presumably a function of the alignment of the rod-like aggregates along their long axis in the direction of flow.f " The shear thinning flow process can be accompanied by an apparent yield stress. Viscoelastic behavior has also been reported. ... 

Many microbial polysaccharides show pseudoplastic flow, also known as shear thinning. When solutions of these polysaccharides are sheared, the molecules align in the shear field and the effective viscosity is reduced. This reduction of viscosity is not a consequence of degradation (unless the shear rate exceeds 105 s 1) since the viscosity recovers immediately when die shear rate is decreased. This combination of viscous and elastic behaviour, known as viscoelasticity, distinguishes microbial viscosifiers from solutions of other thickeners. Examples of microbial viscosifiers are ... 

Many polymers form shear-thinning solutions in water. The molecules are generally long and tend to be aligned and to straighten out in a shear field, and thus to offer less resistance to flow. Such solutions are sometimes viscoelastic and this effect may be attributable to a tendency of the molecules to recover their previous configuration once the stress is removed. Molten polymers are usually viscoelastic. 

Perhaps the most important and striking features of high internal phase emulsions are their rheological properties. Their viscosities are high, relative to the bulk liquid phases, and they are characterised by a yield stress, which is the shear stress required to induce flow. At stress values below the yield stress, HIPEs behave as viscoelastic solids above the yield stress, they are shear-thinning liquids, i.e. the viscosity varies inversely with shear rate. In other words, HIPEs (and high gas-fraction foams) behave as non-Newtonian fluids. 

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. 

Isothermal draw resonance is found to be independent of the flow rate. It occurs at a critical value of draw ratio (i.e., the ratio of the strand speed at the take-up rolls to that at the spinneret exit). For fluids that are almost Newtonian, such as polyethylene terephthalate (PET) and polysiloxane, the critical draw ratio is about 20. For polymer melts such as HDPE, polyethylene low density (LDPE), polystyrene (PS), and PP, which are all both shear thinning and viscoelastic, the critical draw ratio value can be as low as 3 (27). The maximum-to-minimum diameter ratio decreases with decreasing draw ratio and decreasing draw-down length. 

A viscoelastic fluid has the appearance of a solid body it deforms and wholly recovers below t0, and only partly recovers above t0. From 0 to t0, the fluid undergoes an elastic conformational transition above t0, the fluid undergoes an irreversible transition, whence the mass begins to flow toward a new equilibrium position. Carrageenan-water-polyol systems have been suggested to be industrially useful in consideration of their significant t0 followed by shear-thinning (Tye, 1988). 

It has been demonstrated that two types of vortices - viscous corner vortex and viscoelastic lip vortex - exist separately or simultaneously upstream of the contraction. It has also been shown whether they are stable or xmstable. The role of shear-thinning and elasticity in the development of upstream recirculations has also been demonstrated. It should be noted that in a previous study [36], a correlation was established between the development of these vortices, the velocity profile along the upstream axis of flow and the elongational properties of the fluid. 

The behavior of lyotropic crystals is usually non-Newtonian. Lamellar liquid crystals show thixotropic or negative thixotropic behavior,but shear thinning/thixotropy is more common. The shear thinning effect is ascribed to orientation of the liquid crystalline domains and some structural breakdown. Viscoelasticity has also been observed. If the concentration of the surfactant molecules is, however, low and close to the CMC, Newtonian flow will be observed. ... 

Many important coating processes are of liquids that are not Newtonian, and so the effects of non-Newtonian rheology on flow between rolls is of great interest. The code used here has been applied to the simplest non-Newtonian model, namely the purely viscous, shear-thinning fluid. Viscoelasticity, though also important, is more difficult to treat and is not considered here. 

As discussed in Sect. 4, in the fluid, MCT-ITT flnds a linear or Newtonian regime in the limit y 0, where it recovers the standard MCT approximation for Newtonian viscosity rio of a viscoelastic fluid [2, 38]. Hence a yrio holds for Pe 1, as shown in Fig. 13, where Pe calculated with the structural relaxation time T is included. As discussed, the growth of T (asymptotically) dominates all transport coefficients of the colloidal suspension and causes a proportional increase in the viscosity j]. For Pe > 1, the non-linear viscosity shear thins, and a increases sublin-early with y. The stress vs strain rate plot in Fig. 13 clearly exhibits a broad crossover between the linear Newtonian and a much weaker (asymptotically) y-independent variation of the stress. In the fluid, the flow curve takes a S-shape in double logarithmic representation, while in the glass it is bent upward only. 

We can see that Eqs. (2 101) (2-104) are sufficient to calculate the continuum-level stress a given the strain-rate and vorticity tensors E and SI. As such, this is a complete constitutive model for the dilute solution/suspension. The rheological properties predicted for steady and time-dependent linear flows of the type (2-99), with T = I t), have been studied quite thoroughly (see, e g., Larson34). Of course, we should note that the contribution of the particles/macromolecules to the stress is actually quite small. Because the solution/suspension is assumed to be dilute, the volume fraction is very small, (p 1. Nevertheless, the qualitative nature of the particle contribution to the stress is found to be quite similar to that measured (at larger concentrations) for many polymeric liquids and other complex fluids. For example, the apparent viscosity in a simple shear flow is found to shear thin (i.e., to decrease with increase of shear rate). These qualitative similarities are indicative of the generic nature of viscoelasticity in a variety of complex fluids. So far as we are aware, however, the full model has not been used for flow predictions in a fluid mechanics context. This is because the model is too complex, even for this simplest of viscoelastic fluids. The primary problem is that calculation of the stress requires solution of the full two-dimensional (2D) convection-diffusion equation, (2-102), at each point in the flow domain where we want to know the stress. 

Temperature and Viscosity. The operating temperature can have a beneficial effect on flux primarily as a result of a decrease in viscosity.f There is an additional benefit for shear thinning viscoelastic fluids, where the viscosity reduces with an increase in shear (i.e., cross-flow velocity). Typical examples are clarification of fermentation broths and concentration of protein solutions. l l It must be noted that for most fermentation and biotechnology related applications, temperature control is necessary for microbial survival and/or for product stability (e.g., antibiotics, enzymes, proteins and other colloidal materials). 

The deformability of the viscoelastic drops in Newtonian matrix was studied in the convergent slit flow. Both, the experimental observations and the boundary element method computations were carried out. It was reported that deformation of the Boger fluid drop, was quite low —about 1/3 of that recorded for the deformability of a strongly shear-thinning, viscoelastic solution. The latter drops showed deformability similar to these observed for Newtonian drops of similar viscosities. 

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