Steady-shear flow curves for suspensions and solutions measured under equilibrium conditions may exhibit a variety of behaviors over a limited range of shear rates. Additionally, some materials may exhibit more than one distinct behavior over different shear rate regions of the same flow curve. Several types of behavior can be classified according to their characteristic shape. The following classification system covers the six most frequently encountered flow types as illustrated in the accompanying graph. (Figure 2).
Figure 2 Identification of flow curves based on their characteristic shape.
1. Newtonian Differential viscosity and coefficient of viscosity are constant with shear rate.
2. shear-thickening Differential viscosity and coefficient of viscosity increase continuously with shear rate.
3. shear-thinning [pseudoplastic] Differential viscosity and coefficient of viscosity decrease continuously with shear rate. No yield value.
4. shear thinning [pseudoplastic] with yield response Differential viscosity and coefficient of viscosity decrease continuously with shear rate once the apparent yield stress, app, has been exceeded.
5. Bingham plastic (ideal) Obeys the Bingham relation ideally. Above the Bingham yield stress ( B> in Figure 2) the differential viscosity is constant and is called the plastic viscosity, while the coefficient of viscosity decreases continuously to some limiting value at infinite shear rate.
6. Bingham plastic (non-ideal) Above the apparent yield stress the coefficient of viscosity decreases continuously, while the differential viscosity approaches a constant value with increasing shear rate. Extrapolation of the flow curve from the linear, high shear rate region ( plastic region) to the stress axis gives the apparent Bingham yield stress (B* in Figure 2.). The differential viscosity in the linear region in termed the plastic viscosity