Unlike Newtonian fluids, non-Newtonian fluids possess shear-rate dependent viscosities. In addition to shear-rate dependent viscosities, non-Newtonian
fluids also exhibit elastic stresses when subjected to high shear rates. The usefulness of the elastic response varies with application.
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At low shear rates, i.e., near at rest conditions, non-Newtonian fluids exhibit high viscosities that are relatively insensitive to shear rate and characterized by zero shear viscosity. The zero shear viscosity is known to be highly sensitive to the molecular weight and concentration of the rheological additives. The rates of deformation associated with this region include sedimentation and levelling forces, and one can tailor the zero shear viscosity to combat these forces. At moderate shear rates the decrease in viscosity versus shear rate helps when pouring and pumping these fluids. At high shear rates it is found that a second Newtonian plateau in viscosity is reached usually characterised by the so-called infinite viscosity. The shear forces in this area are close in magnitude to forces developed during rubbing and spraying exercises. The low viscosities exhibited by the rheological additives in this region imply low resistance to rubbing and thus a smooth sensation of the substance during its application.

Source: Handbook of Cosmetic Science and Technology - André O. Barel, Marc Paye, Howard I. Maibach

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