Rheology shear-thinning fluids

Rheology concerns the study of the deformation and flow of soft materials when they respond to external stress or strain. If the ratio of its shear stress and shear rate is a straight line, the material is termed Newtonian otherwise, it is termed non-Newtonian (Figure 4.3.2(a)). As the slope of the curve is the viscosity rj, a shear-thinning fluid exhibits a reduced viscosity as the shear stress increases, whereas a shear-... 

Dilatant Fluids. Dilatant fluids or shear-thickening fluids are less commonly encountered than pseudoplastic (shear-thinning) fluids. Rheological dilatancy refers to an increase in the apparent viscosity with increasing shear rate (3). In many cases, viscometric data for a shear-thickening fluid can be fit by using the power law model with n > 1. Examples of fluids that are shear-thickening are concentrated solids suspensions. 

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. 

Soft glasses are known to exhibit remarkable nonlinear shear rheology. They are yield-stress fluids that respond either like an elastic solid when the applied stress is zero or below the yield stress, or a like a viscoelastic fluid when a stress greater than the yield value of the material is applied [185]. Above their yield stresses, soft glasses are shear thinning fluids and very often the shear stress increases with the shear rate raised to the one-half power. This is well documented for the case of concentrated emulsions [102, 182, 186], microgel suspensions [31], and multilamellar... 

Experiments were conducted at room temperature. Water was used as the Newtonian fluid. Non-Newtonian liquids included aqueous solutions ofXanthan gum (Keltrol T, Kelco-Merck) as shear-thinning fluids. Solutions were prepared at constant ionic strength by adding 0.1% (w/v) NaCl. These fluids were selected because they had similar rheological properties to several anaerobic media [6-9] and they are clear fluids that allowed flow pattern visualizations. 

Non-Newtonian behavior of complex fluids is usually governed by various constitutive laws which relate the viscosity of liquids to the rate of shear. The power-law constitutive model is used in most instances due to its ability to predict rheological behaviors of a wide range of non-Newtonian liquids. The power-law model is characterized with a flow behavior index, n, and a flow consistency index, m. Specifically, n=l corresponds to Newtonian fluids whose viscosity is constant, n< corresponds to shear-thinning fluids whose viscosity decreases with increasing the rate of shear, and n>l... 

The discussion in this section is confined to the behaviour of inelastic, shear thinning fluids in porous media. Xanthan biopolymer is taken as the main example, and virtually all of the papers on the flow of this polymer through porous media are related to its importance in oil recovery. Work on the in-situ rheology of xanthan has been reported for flow through sandstone cores, sandpacks, bead packs and other unconsolidated material. 

This model has two parameters or material constants m = consistency and n = flow index. For shear thinning fluids, n is less than 1. Obviously, for n = 1, the model reduces to a Newtonian fluid. Table 10 shows that reducing the particle size increases the consistency and decreases the flow index of the dispersion. This change in rheology is explained as follows ... 

When turbulence takes over, the non-Newtonian rheology plays a much less important role in determining how the liquid will flow. In fact, there is not likely to be too much error by treating shear-thinning fluids... 

Eq. 5.8 gives a model developed by Carreau29,30 describing the complete rheological behavior of a shear-thinning fluid. The parameters K and r,. are determined by fitting the model to ex-... 

Adhesive pastes are non-Newtonian fluids whose viscosity depends upon temperature, time, and shear rate. They are applied to the substrates by means of either stamping (pin transfer), screen printing, or dispensing. The performance of the last technique depends on a key factor the paste rheology. There exists at least six semi-empirical models to describe the rheological response of non-Newtonian fluids [4]. For shear-thinning fluids (thixotropic materials), it has been reported... 

Many materials are conveyed within a process facility by means of pumping and flow in a circular pipe. From a conceptual standpoint, such a flow offers an excellent opportunity for rheological measurement. In pipe flow, the velocity profile for a fluid that shows shear thinning behavior deviates dramatically from that found for a Newtonian fluid, which is characterized by a single shear viscosity. This is easily illustrated for a power-law fluid, which is a simple model for shear thinning [1]. The relationship between the shear stress, a, and the shear rate, y, of such a fluid is characterized by two parameters, a power-law exponent, n, and a constant, m, through... 

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