Shear thinning system

Figure 20.3 Viscosity versus shear rate for a shear thinning system.

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. 

Pseudoplastic a system where the shear rate increases faster than the applied stress also known as shear-thinning system. 

For most practical suspensions (with > 0.1 and containing thickeners to reduce sedimentation) a plot of viscosity depends on the applied shear rate). The most common flow curve is shown in Figure 7.42 (usually described as a pseudo-plastic or shear thinning system). In this case, the viscosity decreases with increasing shear rate, reaching a Newtonian value above a critical shear rate. 

There are usually oil-in-water (0/W) emulsions that are formulated in such a way (see section on cosmetic emulsions) to give a shear thinning system. The emulsion will have a high viscosity at low shear rates (0.1 s ) in the region of few hundred Pa s, but the viscosity decreases very rapidly with increasing shear rate, reaching values of few Pa s at shear rates greater than 1 s h... 

The rheological properties of a cosmetic emulsion that need to be achieved depend on the consumer perspective, which is very subjective. However, the efficacy and aesthetic qualities of a cosmetic emulsion are affected by their rheology. For example, with moisturizing creams one requires fast dispersion and deposition of a continuous protective oil film over the skin surface. This requires a shear thinning system (see below). 

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