Fluids for which the shearing stress is not linearly related to the rate of shearing strain are designated as non-Newtonian fluids. Although there is a variety of types of non-Newtonian fluids, the simplest and most common are shown in Figure.
The slope of the shearing stress vs. rate of shearing strain graph is denoted as the apparent viscosity, For Newtonian fluids the apparent viscosity is the same as the viscosity and is independent of shear rate.
For shear thinning fluids the apparent viscosity decreases with increasing shear rate the harder the fluid is sheared, the less viscous it becomes. Many colloidal suspensions and polymer solutions are shear thinning. For example, latex paint does not drip from the brush because the shear rate is small and the apparent viscosity is large. However, it flows smoothly on to the wall because the thin layer of paint between the wall and the brush causes a large shear rate and a small apparent viscosity. For shear thickening fluids the apparent viscosity increases with increasing shear rate the harder the fluid is sheared, the more viscous it becomes. Common examples of this type of fluid include water-corn starch mixture and water-sand mixture (“quicksand”). Thus, the difficulty in removing an object from quicksand increases dramatically as the speed of removal increases.
A mixture of water and corn starch, when placed on a flat surface, flows as a thick, viscous fluid. However, when the mixture is rapidly disturbed, it appears to fracture and behave more like a solid. The mixture is a non-Newtonian shear thickening fluid which becomes more viscous as the shearing rate is suddenly increased through the rapid action of the spoon.
The other type of behavior indicated in Figure is that of a Bingham plastic, which is neither a fluid nor a solid. Such material can withstand a finite shear stress without motion (therefore, it is not a fluid), but once the yield stress is exceeded it flows like a fluid (hence, it is not a solid). Toothpaste and mayonnaise are common examples of Bingham plastic materials.
References of texts and movies:
Fundamental of fluid mechanics by: Bruce R. Munson and Donald F. Young
A movie from CD of aforementioned book (V1.4)
The Big Bang Theory (TBBT) Serial: Season 3, Episode 20, The Barbarian Sublimation