Shear-thickening fluid, rheology

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. 

Other examples of structured fluids are fluids, where forces depend nonlinearly on the rate of deformation, such as shear-thinning or shear-thickening fluids. The mechanical behavior of these structured fluids is studied in the field of rheology. 

In wet grinding the power consumption is generally about 30 per cent lower than that for dry grinding and, additionally, the continuous removal of product as it is formed is facilitated. The rheological properties of the slurry are important and the performance tends to improve as the apparent viscosity increases, reaching an optimum at about 0.2 Pa.s. At very high volumetric concentrations (ca. 50 volume per cent), the fluid may exhibit shear-thickening behaviour or have a yield stress, and the behaviour may then be adversely affected. 

Laun, H. M. Bung, R., and Schmidt, F. 1991. Rheology of extremely shear thickening polymer dispersions passively viscosity switching fluids. / Rheol. 35 999-1034. 

Simple classifications of fluids can be made on the basis of their rheological profiles. Figure 3.78 shows the (a) shear stress and (b) viscosity profiles for various systems. From Figure 3.78 one may define the following systems. Newtonian systems have a constant viscosity with respect to shear rate. Dilatant (or shear-thickening) systems have a viscosity that increases with respect to shear rate. Pseudo-plastic (or shear-thinning) systems have a viscosity that decreases with respect to shear rate. Yield-stress materials are materials that have an initial structure that requires a finite stress before deformation can occur. The stress that initiates deformation is defined as the yield stress. 

Another aspect of cement slurry stability is the stability under dynamic conditions. Dynamic conditions are usually more severe than static ones because cement slurries are shear-thinning. This is a problem in the laboratory as the solid particles may settle while the fluid is being sheared (thickening time, rheology), and also in the field especially if the well is deviated from vertical. But there is currently no standard test in the industry to evaluate the stability of cement slurries under dynamic conditions. 

Hu, Y, Wang, S. Q., and Jamieson, A. M., Rheological and flow birefringence studies of a shear-thickening complex fluid a surfactant model system, /. RheoL, 37,531-546 (1993b). 

By plotting these data on linear and logarithmic scales, ascertain the type of fluid behaviour, e.g. Newtonian, or shear-thinning, or shear-thickening, etc. Also, if the liquid is taken to have power-law rheology, calculate the consistency and flow-behaviour indices respectively for this liquid. 

Concentrated dispersions may be shear thickening, as opposed to the shear thinning of dilute polymer solutions. Some materials, such as latex paints, tend to form a structure. As the structure breaks down with shearing action, the viscosity decreases. Such materials are thixotropic. Some fluids have a yield stress. A thorough characterization of the rheology may require a number of different measurements. 

For dilutant fluids, n > 1. Rheological dilatancy refers to increasing viscosity with increasing shear rate. Therefore, these fluids are also called shear-thickening. Examples include whipped cream and starch slurries. They are rare in industrial practice. 

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