Newtonian behaviour, disperse suspensions

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (02.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation (02.6.2) can be used to define an apparent viscosity, happ, at a given shear rate. If q pp decreases witli increasing shear rate, tire dispersion is called shear tliinning (pseudoplastic) if it increases, tliis is known as shear tliickening (dilatant). The latter behaviour is typical of concentrated suspensions. If a finite shear stress has to be applied before tire suspension begins to flow, tliis is known as tire yield stress. The apparent viscosity may also change as a function of time, upon application of a fixed shear rate, related to tire fonnation or breakup of particle networks. Thixotropic dispersions show a decrease in q, pp with time, whereas an increase witli time is called rheopexy. 

The categorizations just presented are helpful in describing the rheological properties of many emulsions, foams, and suspensions. It should be remembered, however, that some dispersions are not well described by any single category. Some dispersions exhibit Newtonian behaviour at low shear rates, shear thinning at moderate shear rates,... 

Investigations of the rheological properties of disperse systems are very important both from the fundamental and applied points of view (1-5). For example, the non-Newtonian and viscoelastic behaviour of concentrated dispersions may be related to the interaction forces between the dispersed particles (6-9). On the other hand, such studies are of vital practical importance, as, for example, in the assessment and prediction of the longterm physical stability of suspensions (5). 

Using model concentrated suspensions of polyvinyl chloride and titanium dioxide particles in a Newtonian polybutene fluid, small amplitude oscillatory shear and creep experiments were described [2]. It was shown that the gel-like behaviour at very small strain, and strain hardening at a critical strain, are caused by particle interactions and the state of particle dispersion. 

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