Newtonian fluid behaviour

Boger, D.V. Demonstration of upper and lower Newtonian fluid behaviour in a pseudo-plastic fluid. Nature 1977,265, 126-128. 

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. 

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. 

In these equations, m and n are two empirical curve-fitting parameters and are known as the fluid consistency coefficient and the flow behaviour index respectively. For a shear-thinning fluid, the index may have any value between 0 and 1. The smaller the value of n, the greater is the degree of shear-thinning. For a shear-thickening fluid, the index n will be greater than unity. When n = 1, equations (1.12) and (1.13) reduce to equation (1.1) which describes Newtonian fluid 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. 

Note that in the limit of ti/2 oo, i.e. for Newtonian fluid behaviour, this equation reduces to the Hagen-Poiseuille equation. 

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