Not a typical fluid. Unlike a Newtonian fluid, which displays liquid behavior, a non-Newtonian fluid has properties of a liquid and of a solid. Under certain conditions, a non-Newtonian fluid flows as a liquid and under other conditions, it exhibits elasticity, plasticity and strength similar to a solid. In addition, unlike Newtonian fluids, the viscosity of many non-Newtonian fluids varies with shear rate.
Four classes of non-Newtonian fluids depend on how the fluid viscosity—a measure of a fluid’s ability to resist flow—varies in response to the duration and magnitude of applied shear rate. The viscosity of:
Most successful drilling fluids are non-Newtonian and exhibit behaviors that are described by rheological mathematical models of shear stress, or resistance, as a function of shear rate. In the Bingham plastic model, flow will not begin until the shear stress attains a minimum value, the yield stress, after which the flow is similar to that of a Newtonian fluid because the viscosity is constant and does not vary with shear rate.
Pseudoplasticity, or shear thinning, is a non-Newtonian behavior that is desirable for drilling fluids. Power-law and Herschel-Bulkley models describe pseudoplastic behavior, in which the slope—the viscosity—of the shear stress versus shear rate curve decreases as the shear rate decreases.