Silica spheres shear thinning

Van der Werff and de Kruif (1989) examined the scaling of rheological properties of a hard-sphere silica dispersion (sterically stable monodisperse silica in cyclohexane) with particle size, volume fraction and shear rate. The shear-thinning behaviour was found to scale with the Peclet number Pe = 6nt]sa yl k-QT), or the ratio of shear time to structure-build-up time, where a is the particle radius, is the viscosity of the solution, y is the shear... 

Figure 19-11. Reduced viscosities as a function of the reduced shear stress of colloidal silica suspensions (diameter of 100 nm) in the presence of addedpolymer (polystyrene). The solvent used is decalin which is a near theta solvent for polystyrene. The size ratio of the polymer radius of gyration to the colloid radius (Rg/R) is 0.02S. The colloid volume fraction ((f>) is kept fixed at 0.4. In the absence of added polymer (Cp/c = 0), the particles behave as hard spheres and as more polymer is added to the system, the particles begin to feel an attraction. The colloid-polymer suspensions at (p of 0.4 shear thin between a zero rate viscosity of r o and a high shear rate plateau viscosity r]x,. The shear thinning behavior (in the absence and presence of polymer) is well captured by equation (19-10) with n = 1.4. Note rjo, rjao and cTc are functions of volume fraction and strengths of attraction but weakly dependent on range of attraction (Shah, 2003c Rueb, 1997).

Jones, et al. examined 50 nm silica spheres coated with covalently-bound stearyl alcohol dissolved in Shellsol T(55). Viscosities were determined with Ubbelohde viscometers and with three different cone and plate instruments. Sphere volume fractions were taken as high as 0.635, corresponding to T]r as large as 9.2 10 . Shear thinning was apparent at concentrations above 0.4. Systems with (p > 0.64 could not be taken into the low-shear limit in which 17 (/c) becomes independent from /c, so the low-shear rj remains indeterminate at these very large concentrations. 

There are limited measurements of the viscoelastic properties of hard-sphere colloids at elevated shear rates, stresses, and frequencies, de Kruif, et al. report rir of silica spheres in the small and large shear-rate limits, as functions of (53). At smaller concentrations, rjr depends but weakly on /c. Above

shear thinning becomes apparent, de Kruif, et al. propose that r]r diverges 2 — 4>/4>m), with 4>m being 0.71 or 0.63 in the large and small shear limits. Jones, et al. also find weak shear thinning for (p > 0.395, the extent of shear thinning increases quite substantially for volume fractions between 0.59 and 0.60(55). 

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,... 

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