ABSTRACT
An external stress applied to a portion of a fluid, as illustrated in Figure 10.1, will introduce a movement of the molecules of the affected part of the fluid in the direction of the applied stress. The moving molecules will interact with the neighboring molecules, which will start moving too, but with a lower velocity than that of the molecules exposed to the stress. The dynamic viscosity, η, is defined as
η τ
= ∂ ∂
y
(10.1)
where τxy = applied shear stress (force per area unit = F/A) vx = velocity of fluid in x-direction
∂ ∂ v
y x = shear rate
If the viscosity is independent of shear rate, the fluid is said to be Newtonian in behavior. Figure 10.2 illustrates Newtonian and three kinds of non-Newtonian flow behavior. Pseudoplastic and dilatant fluids exhibit a nonlinear relationship between shear rate and shear stress. The viscosity of a pseudoplastic fluid decreases with increasing shear rate, whereas the opposite is the case for a dilatant fluid. A Bingham plastic fluid is similar to a Newtonian fluid in the sense that there is a linear relationship between shear stress and shear rate. However, a Bingham plastic fluid differs by requiring a finite shear stress (pressure) to initiate a flow.
