Shear-thinning fluid properties

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). 

In the case of fluids without yield stress, viscous and viscoelastic fluids can be distinguished. The properties of viscoelastic fluids lie between those of elastic solids and those of Newtonian fluids. There are some viscous fluids whose viscosity does not change in relation to the stress (Newtonian fluids) and some whose shear viscosity T] depends on the shear rate y (non-Newtonian fluids). If the viscosity increases when a deformation is imposed, we define the material as a shear-thickening (dilatant) fluid. If viscosity decreases, we define it as a shear-thinning fluid. 

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. 

During cardiopulmonary bypass, addition of fluids such as anticoagulants leads to a reduction in the hematocrit or percentage or the total blood volume that is made up of red blood cells. Thus the elastic properties of blood do not affect blood flow in a BO [48]. Further, since the hematocrit of the blood is low, cell-cell interactions are likely to be less important. In modem BOs, the average relative shear stress on the blood is about 5-30 Pa [20]. Under these conditions, blood may be modeled as a shear-thinning fluid. Consequently, a Newtonian blood analogue fluid cannot model the variation of blood viscosity with shear rate. 

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. 

For shear-thinning fluids, the apparent shear rate at the wall is less than the true shear rate, with the converse applying near the centre of the tube [Laim, 1983]. Thus at some radius, x R, the true shear rate of a fluid of apparent viscosity /r, equals that of a Newtonian fluid of the same viscosity. The stress at this radius, is independent of fluid properties and thus the true viscosity... 

The power law exponent, n, is unity for a Newtonian fluid and less than unity for a shear-thinning fluid. One of the central questions of polymer physics concerns the molecular basis for the constitutive equations. Because NMR is so sensitive to molecular dynamical parameters, the simultaneous mapping of velocity profiles and molecular properties such as the polymer self-diffusion coefficient by means of the dynamic NMR microscopy technique offers an effective test of much molecular models. 

Non-Newtonian properties can have several different influences on fluids. Eor this work, we limit ourselves to simulating time-independent shear-thinning effects. The most basic way of modeling shear-thinning fluids is using an Ostwald/de Wale-type power law model. Eor our early investigations, we simulated jets from an... 

Non-Newtonian fluids vary significantly in their properties that control flow and pressure loss during flow from the properties of Newtonian fluids. The key factors influencing non-Newtonian fluids are their shear thinning or thickening characteristics and time dependency of viscosity on the stress in the fluid. 

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... 

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. 

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