Non-Newtonian fluid behaviour

For a Newtonian fluid in simple shearing motion, the deviatoric normal stress components are identically zero, i.e. 

the complete definition of a Newtonian fluid is that it not only possesses a constant viscosity but it also satisfies the condition of equation (1.9), or simply that it satisfies the complete Navier-Stokes equations. Thus, for instance, the so-called constant viscosity Boger fluids [Boger, 1976 Prilutski et al., 1983] which display constant shear viscosity but do not conform to equation (1.9) must be classed as non-Newtonian fluids. 

A non-Newtonian fluid is one whose flow curve (shear stress versus shear rate) is non-linear or does not pass through the origin, i.e. where the apparent viscosity, shear stress divided by shear rate, is not constant at a given temperature and pressure but is dependent on flow conditions such as flow geometry, shear rate, etc. and sometimes even on the kinematic history of the fluid element under eonsideration. Such materials may be conveniently grouped into three general elasses  

This classification scheme is arbitrary in that most real materials often exhibit a combination of two or even all three types of non-Newtonian features. Generally, it is, however, possible to identify the dominant non-Newtonian characteristic and to take this as the basis for the subsequent process calculations. Also, as mentioned earlier, it is convenient to define an apparent viscosity of these materials as the ratio of shear stress to shear rate, though the latter ratio is a fimction of the shear stress or shear rate and/or of time. Each type of non-Newtonian fluid behaviour will now be dealt with in some detail. 

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The materials etKountered in the industrial world, and indeed in the world of everyday life, often show characteristics of both liquids and solids, and latexes are no exception. Latexes may dononstrate Newtonian behaviour at very low concentrations, but in the range of concentrations and und the conditions in which they are used in industry, they often show both non-Newtonian fluid behaviour and viscoelastic behaviour. 

Similarly, since much has been written about the importance of the measurement of rheological data in the same range of shear or deformation rates as those likely to be encountered in the envisaged application. Table 1.3 gives typical orders of magnitudes for various processing operations in which non-Newtonian fluid behaviour is likely to be significant. 

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