phenomenon is calledshear thickening, which widely exists in the dispersing
systems like latexes, suspensions, filled polymers, flour/sugar solutions, etc.
Curve p represents thepseudo-plastic fluid, whose viscositydecreases with the
increase of shear rateg^0. Such a phenomenon is calledshear thinning, which widely
exists in the oriented flows of most linear polymers, adhesives, lubricants, soap
fluids, paper pulp, coatings, etc. Curve B represents theBingham fluid(Bingham
1916 ). A yielding stressscis required to initiate the Newtonian-fluid behavior of a
Bingham fluid. The real Bingham fluids often contain a networking molecular
structure, and the viscosity changes with the initiation of flow, such as toothpaste,
grease, cement, coal pulp, etc.
If the shear rates are constants, the non-Newtonian fluids can also be classified
according to their viscosity dependence on time. This classification has been widely
applied to describe the rheological characteristics of coatings. For the development
of deformation, the time evolution corresponds to the effect of the increase of shear
rate. Three typical cases occur with the time evolution: thethixotropicfluids exhibit
the decrease of viscosity, corresponding to pseudo-plastic fluids; therheopectic
fluids exhibit the increase of viscosity, corresponding to dilatant fluids; while the
viscoelasticfluids exhibit partial recovery of the deformation of pseudo-plastic
fluids after the removal of the stress. Since polymers can perform a large scale of
elastic deformation, this character appears extremely significant.
De Waele and Ostwald proposed a unified power-law equation, as
s¼Kg^0 n (7.7)whereKis the temperature-sensitive parameter, andnis the characteristic index
determined by the structure of the fluids:n¼1 for the Newtonian fluids;n>1 for
dilatant fluids, andn<1 for pseudo-plastic fluids. The description to the complex
Fig. 7.4 Illustration of several conventional Non-Newtonian fluids deviating from the Newtonian
fluids.Nrepresents Newtonian fluids,drepresents dilatant fluids,prepresents pseudo-plastic
fluids, andBrepresents Bingham fluids
7.1 Introduction to Rheology 131