Newtonian versus Non-Newtonian
Simply, Newtonian fluids stay just as thick no matter how hard or fast you stir it. Technically, Newtonian fluids are defined as fluids whose viscosity stays constant regardless of applied shear stress and shear rate. Some examples include water, gasoline, milk, and honey*. Most fluids, however, are non-Newtonian.
The mayonnaise on your sandwich, the ketchup for your fries, or the blood in your body are examples of non-Newtonian fluids. These are fluids whose viscosity do depend on how hard you push them or how quickly you push them. There are several types of non-Newtonian fluids.
*Honey attributes non-Newtonian behavior when it crystallizes. Stirring it will thin it out, but once it is a liquid, it exhibits Newtonian behavior.
The image on the right, courtesy of RheoSense, displays the behavior of various non-Newtonian fluids.
|Rheopectic||Viscosity increases with time, i.e., longer stirring yields a thicker fluid. Leave them and they will thin out.||Shear rate||Cream will thicken the longer you stir it.|
|Thixotropic||Over time, viscosity decreases, i.e., longer stirring yields a thinner fluid. Leave them and they will thicken.||Shear rate||Nail polish will thin out the longer you shake the bottle.|
|Pseudoplastic (Shear thinning)||Higher stress yields lower viscosity, i.e., hit it hard and it will thin out.||Stress||Ketchup needs an initial force to flow out of the bottle. The harder you squeeze, the runnier it comes out.|
|Dilatant (Shear thickening)||Higher stress yields higher viscosity, i.e., hit it hard and it will thicken.||Stress||Oobleck: hit it with a hammer and it stays solid. Let the hammer sit on it, and it will sink.|
|Bingham plastic||They won't flow at all until a high enough stress is applied. Then, the viscosity is independent of shear or shear rate.||Stress (initially)||Toothpaste tubes need an initial squeeze to get the toothpaste out. However, squeezing it harder will not make it flow any easier.|
|Bingham pseudoplastic||They are just like Bingham plastics except that they thin out with a higher shear rate.||Stress||Mayonnaise will not flow from a bottle until it is squeezed. The harder you squeeze it, the thinner it will come out.|