Definition - What does Non-Newtonian Fluid mean?
A Non-Newtonian Fluid is a liquid that behaves like a solid and remains in a semi-solid or highly viscous state. Such fluids do not follow the Newton’s Law of viscosity and thus the name non-Newtonian.
As per rheology (a branch of physics that deals with the study of the flow of liquid matter and the deformation of solid materials), non-Newtonian fluids have varying viscosity and can be deformed from one shape to another when force or stress is applied on them.
Petropedia explains Non-Newtonian Fluid
The most common and simple example of a non-Newtonian fluid is a corn starch dissolved in water. The viscosity of non-Newtonian fluids is very low and thus, these fluids remain in a semi-solid state.
If we consider a Newtonian fluid, the relation between the shear stress and shear strain would be linear and the coefficient of viscosity remains constant; however, in the case of a non-Newtonian fluid there’s no linear relation between shear stress and strain, it is different and time dependent because of which the coefficient of viscosity would not be constant and cannot be defined.
Drilling fluids sometimes exhibit the properties of non-Newtonian fluids due to shear thinning behavior in which the viscosity of the drilling fluids decreases under shear strain. Since shear strain is inversely proportional to shear stress, it means that whenever the viscosity of drilling fluids decreases, the stress increases and thus, the shear rate also increases.