"Parallel" linear combination of elastic and viscous effects Comparison of non-Newtonian, Newtonian, and viscoelastic properties Types of non-Newtonian behavior Summary Ĭlassification of fluids with shear stress as a function of shear rate. The properties are better studied using tensor-valued constitutive equations, which are common in the field of continuum mechanics. They are best studied through several other rheological properties that relate stress and strain rate tensors under many different flow conditions-such as oscillatory shear or extensional flow-which are measured using different devices or rheometers. Therefore, a constant coefficient of viscosity cannot be defined.Īlthough the concept of viscosity is commonly used in fluid mechanics to characterize the shear properties of a fluid, it can be inadequate to describe non-Newtonian fluids. The fluid can even exhibit time-dependent viscosity. In a non-Newtonian fluid, the relation between the shear stress and the shear rate is different. In a Newtonian fluid, the relation between the shear stress and the shear rate is linear, passing through the origin, the constant of proportionality being the coefficient of viscosity. Some non-Newtonian fluids with shear-independent viscosity, however, still exhibit normal stress-differences or other non-Newtonian behavior. Most commonly, the viscosity (the gradual deformation by shear or tensile stresses) of non-Newtonian fluids is dependent on shear rate or shear rate history. Many salt solutions and molten polymers are non-Newtonian fluids, as are many commonly found substances such as custard, toothpaste, starch suspensions, corn starch, paint, blood, melted butter, and shampoo. Ketchup, for example, becomes runnier when shaken and is thus a non-Newtonian fluid. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, that is, it has variable viscosity dependent on stress.
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