The resistance which arises from the lack of slipperiness of the parts of the liquid, other things being equal, is proportional to the velocity with which the parts of the liquid are separated from one another. —Isaac Newton
Sir Isaac Newton described how liquids behave. Today, we call this description Newton’s Law of Viscosity, and it describes how fluids move in response to stress. Take a straw in a glass of water as an example. Suck on the straw, and you pull water from the glass. Suck harder, and the water moves faster. Its viscosity, or resistance to flow, doesn’t change: Everything’s in proportion. Fluids that follow the law of viscosity are Newtonian fluids.
A non-Newtonian fluid is a fluid whose viscosity is variable based on applied stress.
If you punch a bucket full of a shear thickening non-Newtonian fluid [e.g. cornstarch dispersed into water*], the stress introduced by the incoming force causes the atoms in the fluid to rearrange such that it behaves like a solid. Your hand will not go through. If you shove your hand into the fluid slowly, however, it will penetrate successfully. If you pull your hand out abruptly, it will again behave like a solid, and you can literally pull a bucket of the fluid out of its container in this way.
A shear thinning non-Newtonian fluid behaves in the opposite way. In this type, the fluid becomes thinner, rather than thicker, when stress is applied. Also called pseudoplastic, examples of this type of non-Newtonian fluid include ketchup, [wall paint and blood].
When combined with a oscillating plate, non-Newtonian fluids demonstrate other unusual properties, like protruding “fingers” and holes that persist after creating them. An oscillating plate applies stress on a periodic basis, rapidly changing the viscosity of the fluid and putting it in an odd middle ground between a liquid and a solid.