Shear Thickening System

In some cases the very act of deforming a material can cause rearrangement of its microstructure such that the resistance to flow increases with an increase of shear rate. In other words, the viscosity increases with appHed shear rate and the flow curve can be fitted with the power law. Equation (20.3), but in this case n 1. The shear thickening regime extends over only about a decade of shear rate. In almost all cases of shear thickening, there is a region of shear thinning at low shear rates. 

Several systems can show shear thickening, such as wet sand, com starch dispersed in milk and some polyvinyl chloride sols. Shear thickening can be illustrated when somebody walks on wet sand such that some water is squeezed out and the sand appears dry. The deformation applied by the feet causes a rearrangement of the close-packed stracture produced by the water motion, and this process is accompanied by a volume increase (hence the term dilatancy) as a result of sucking in of the water. The process amounts to a rapid increase in the viscosity. 

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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. 

When shear-thickening systems need some time to reestablish o(dy/dt) equilibrium, they are called rheopectic. Thus, rheopexy is the relatively slow increase in due to gradual interaction and structure formation. Rheopexy is a rather rare and not a very relevant phenomenon. 

Certain polymeric systems can become more viscous on shearing ( shear thickening ) due to shear-introduced organization. These systems become more resistant to flow as the crystals form so that the introduction of the shear increases their viscosity. Figure 6.5 shows the viscosity versus strain rate relationship for Newtonian and non-Newtonian fluids, highlighting the differences in their behaviors. 

From the above analysis it follows that the increase in Ti g at smaU v cannot be interpreted as shear thickening. Tribological driving (at positive temperatures) would result in a curve similar to that shown in Fig. 15, in which the linear-response viscosity is obtained at the smallest shear rates. However, as the two model systems mentioned above are athermal, their tribologically determined rjgff would tend to infinity at zero shear rate [216]. 

Watanabe et al (1996) describe the non-linear dynamic rheology of a concentrated spherical silica-particle-filled ethylene glycol/glycerol system. The strain dependence was well described by the BBCZ-type constitutive equation, until the shear-thickening regime. The shear thickening was qualitatively described in relation to the structure of the suspended filler particles. 

Hadjistamov (1999) examined the effect of nanoscale silica on the rheology of silicone oil and uncured epoxy-resin (araldite) systems. Shear thickening and yield-stress-like behaviour were observed and found to be due to a build-up of network structure associated with the nanocomposite phase. 

Deviations of liquids from Newtonian behavior are frequently observed for pharmaceutical and biomedical systems. In these, the relationship between stress and the rate of strain is nonlinear, examples of which include pseudoplastic (shear thinning), dilatant (shear thickening), plastic, and Bingham and Ostwald systems (1,17). Such systems are commonly referred to as non-Newtonian systems. 

The degree of concavity is a measure of the shortness, or butterlike quality, of the dispersion. The system is known as shear thinning (34). Naturally, a system displaying opposite concavity would be called shear thickening. Such terms as thixotropic and pseudoplastic are to be avoided, even though they appear frequently in the literature. 

This chapter is an in-depth review on rheology of suspensions. The area covered includes steady shear viscosity, apparent yield stress, viscoelastic behavior, and compression yield stress. The suspensions have been classified by groups hard sphere, soft sphere, monodis-perse, poly disperse, flocculated, and stable systems. The particle shape effects are also discussed. The steady shear rheological behaviors discussed include low- and high-shear limit viscosity, shear thinning, shear thickening, and discontinuity. The steady shear rheology of ternary systems (i.e., oil-water-solid) is also discussed. 

Here n is the consistency (or power law) index and K is the consistency of the material. When n > 1, the system is shear thickening, whereas... 

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