Newtonian Fluid

A Newtonian fluid is a fluid whose viscosity remains constant and is independent of shear rate. This means that shear stress is proportional to shear rate, and the fluid behaves in a linearly viscous manner. Typical examples include water, air, and many simple oils. The flow of such fluids can be described using the Navier-Stokes equations.
Non-Newtonian fluids such as blood or ketchup, on the other hand, exhibit shear rate-dependent viscosity. In filtration, the rheological properties of the fluid determine whether a filter cake remains stable and how much pressure needs to be applied.