Bingham gels

Rheology has also been used to locate sol-gel transitions in concentrated block copolymer solutions, as described in Chapter 4. Gels exhibit a finite yield stress (i.e. they are Bingham fluids), which can be measured in steady shear experiments. 

Figure 6.14 Bingham yield stress of clay gels as a function ofthe amount of NaCI added. From van Olphen [1], Copyright 1977, Wiley-lnterscience.

At high shear rates, when the gel network is broken down, the dominant viscoelastic contribution comes from floes that break apart and reform rapidly. For such dispersions, at modest particle volume fractions, a typical relationship between steady-state shear stress a and shear rate y is shown in Fig. 7-24 (Friend and Hunter 1971). Note that at the highest shear rates, the y-a relationship appears to be linear, but the extrapolation of this linear relationship to zero shear rate intersects the stress axis at a positive value, as, rather than zero. This intercept is called the Bingham yield stress, derived from the Bingham equation for shear stress (Friend and Hunter 1971) ... 

Slow sedimentation of particles will occur, for example, in an activated sludge or in fine particle catalyst suspensions. For those lands of systems, a homt eneous distribution of solids is characteristic. Here, the liftoff from the vessel bottom as well as the state of a homogeneous suspension can be achieved with a comparably low power input or only slight movement of the liquid. On the other hand, at higher solids concentrations a pseudoplastic flow characteristic of the suspension can occur. As an example, concentrations of only 6% of fibrous material - typically known from paper industry - can lead to this non-Newtonian behavior Frequently observed in suspensions with high solids concentrations is a Bingham plastic behavior. In this case, if a certain amount of shear is not introduced by agitation, the system behaves like an elastic solid body or a gel. 

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