Newtonian vs Non-Newtonian Fluids

Fluids that satisfy

$$\tau =\mu \frac{du}{dy}\text{or}\tau =\left|\mu \frac{du}{dy}\right|$$

where $\mu$ is the viscosity are Newtonian fluids.

Fluids for which the shearing stress is not linearly related to the rate of shearing strain are designated as non-Newtonian fluids.