TABLE OF CONTENTS We have defined a Newtonian fluid as one that has a constant viscosity at a given temperature and pressure irrespective of the way in which it is flowing or being moved. We can also say that the ratio of shear stress to shear rate in such a fluid would have a constant value at constant temperature and pressure. Effectively, all gases approximate to this Newtonian type of flow behavior and most low-viscosity liquids are also Newtonian. That a fluid is either compressible (like a gas) or incompressible (like water) has no significance in deciding whether it is Newtonian or non-Newtonian in flow behavior.
Non-Newtonian fluids show a different apparent viscosity depending on the shear to which they are exposed that is, depending on the way in which they are flowing or being moved. Apparent viscosity for some fluids varies with the magnitude of the shear, and for others it depends also on how long they experience shear. Usually non-Newtonian flow arises in fluids with complex molecular structures (e.g., polymers) and some hydrocarbon crude oils, and where large groups of particles are loosely held together such as in flocculated slurries or suspensions of solids in liquids; pastes and emulsions also usually fall into this category.
To decide whether a fluid is Newtonian or non-Newtonian, we need to know how it behaves in response to shear.
Suppose I now explain that the Newtonian kind of flow behavior is really just a special case, just a way that some fluids behave...
