Suspensions concentrated, shear thinning

Concentrated suspensions can also behave as Newtonian, but more commonly these suspensions show shear thinning or shear thickening behavior. 

Lee, et al. measured viscosity as affected by shear rate for silica sphere suspensions, finding shear thinning at lower shear rates(57). In some but not all systems and volume fractions above 0.5, a reproducible abrupt transition to shear tbickening was found at elevated shear rates. The transition shear rate depended on concentration and temperature. In contrast, Jones, et al plot only a shear thinning region. A possible explanation for this difference is provided by the Peclet number Pe,... 

A highly concentrated suspension of flocculated kaolin in water behaves as a pseudo-homogeneous fluid with shear-thinning characteristics which can be represented approximately by the Ostwald-de Waele power law, with an index of 0.15. It is found that, if air is injected into the suspension when in laminar flow, the pressure gradient may be reduced even though the flowrate of suspension is kept constant, Explain how this is possible in slug flow and estimate the possible reduction in pressure gradient for equal volumetric flowrates of suspension and air. 

As is evident from Eq. (3-20) or (3-21), the Bingham plastic exhibits a shear thinning viscosity i.e., the larger the shear stress or shear rate, the lower the viscosity. This behavior is typical of many concentrated slurries and suspensions such as muds, paints, foams, emulsions (e.g., mayonnaise), ketchup, or blood. 

Variants to this expression and alternative models have also been proposed to describe shear thinning behaviour of concentrated suspensions [12,13]. 

In concentrated suspensions, the particles touch each other. If there is also an attraction between the particles, the suspension may not flow when the shear stress is small it is a solid (Figure C4-14). The stress at which the liquid starts moving is known as the yield stress. Once the liquid yields, it often behaves like a Newtonian liquid with a constant differential viscosity. The behaviour of such Bingham fluids is similar to that of shear thinning fluids ... 

The technological importance of xanthan gum rests principally on its unusual and distinctive properties25 28 29 49,116,251,257-260 in aqueous solution. Some of these properties are (1) remarkable emulsion-stabilizing and particle-suspension ability, (2) low concentrations yield high viscosities, (3) recoverable shear-thinning (extremely large shear dependence of viscosity), (4) little variation in viscosity with temperature under normal conditions of industrial utilization, and (5) gel formation when mixed with certain other, nongelling polysaccharides. 

Problem 6.7(a) (Worked Example) Estimate the first normal stress difference Ni for a suspension of long, thin particles (approximated as spheroids) with p = 100 and L = 0.1/ m, if the solvent viscosity is 1 P, the shear rate y is 100 sec, and the particle concentration is 0 = 0.001, which is in the dilute regime. 

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. 

Shear thinning of concentrated suspensions is typical for submicron particles dispersed in a low viscosity Newtonian fluid.At low shear strain rates. Brownian motion leads to a random distribution of the particles in the suspension, and particle collision will result in viscous behavior. At high shear strain rates, however, particles will arrange in layers, which can slide over each other in the direction of flow. This results in a reduced viscosity of the system in agreement with the principles of shear thinning. A pro-noimced apparent yield stress can be found for shear thinning suspensions, if the Brownian motion is suppressed by electrostatic repulsion forces, which result in three-dimensional crystal-like structures of the particles with low mobility. 

Emulsions are increasingly non-Newtonian, i.e., shear thinning with increasing concentration of the lipophilic phase. The relative effects of droplet size and shape and their distributions are less well studied, compared to solid particle suspensions, but in general, an increase in viscosity is observed for smaller and more monosized droplets. Hence, an increase in energy input, for example, by homogenization, will always result in an increase in viscosity. 

Xanthan forms the most pseudoplastic (instantaneous, reversible shear thinning) solutions of all the gums. This property is due to the stiffness of its molecules and/or intermolecular associations of two or more molecules. In plots of viscosity vs. concentration, there is a Newtonian (non-pseudoplastic) plateau at very low shear rates, which at least, makes its solutions appear to have a yield value (a yield value being the force required to initiate flow). As a result, xanthan is an excellent stabilizer for suspensions and emulsions. 

The flow pattern of the powder mixture through the extruder influences the quality of the shaped article. The rheology of concentrated ceramic suspensions can be divided into four classes ideal plastic, Bingham, shear thinning, and shear thickening (dilant). 

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... 

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