Quick A Newtonian fluid is a fluid whereby the rate of deformation (strain rate) is proportional to the applied shear stress where the constant of proportionality is viscosity.
Nonnewtonian fluids are fluids that DO NOT follow the linear law of Eq2 from the lesson Viscosity and are treated in books on rheology.
Details
Fluids for which the rate of deformation is proportional to the shear stress are called Newtonian fluids. Most common fluids such as water, air, gasoline, and oils are Newtonian fluids. Blood and liquid plastics are examples of non-Newtonian fluids.
The following figure compares some different materials with a newtonian fluid.
A dilatant fluid increases resistance with increasing applied stress. Alternatively, a pseudoplastic fluid decreases resistance with increasing stress. If the thinning effect is very strong the fluid is termed plastic. The limiting case of a plastic substance is one that requires a finite yield stress before it begins to flow. The linear-flow Bingham plastic idealization is shown, but the flow behavior after yield may also be nonlinear. An example of a yielding fluid is toothpaste, which will not flow out of the tube until a finite stress is applied by squeezing.
A further complication of nonnewtonian behavior is the transient effect shown in the following figure:
Some fluids require a gradually increasing shear stress to maintain a constant strain rate and are called rheopectic. The opposite case of a fluid that thins out with time and requires decreasing stress is termed thixotropic. Fluid mechanics typically deals with the analysis of newtonian fluids.
