A fluid whose viscosity does not change even if the force exerted upon it changes is called a "Newtonian fluid," and a fluid whose viscosity changes when the force exerted upon it changes is called a "non-Newtonian fluid."
As a general rule, pure substances are usually Newtonian fluids. Mixtures of two or more substances, however, are almost all non-Newtonian fluids.
Some examples of Newtonian fluids are: water, honey, cooking oil, 'mizuame' (a thick liquid sugar converted from starch), sugar water solution, sodium chloride solution, and alcohol.
Some examples of non-Newtonian fluids are: butter, ketchup, mayonnaise, and yogurt.
Non-Newtonian fluids can be divided into Bingham fluids, pseudoplastic fluids, and dilatant fluids.
Butter can be spread on toast with the force from a knife, but unless a certain amount of force is applied, the butter does not move. The force required to make the butter move is called yield stress, and the value expressing yield stress is the yield value. When a substance has a yield value but keeps a constant viscosity when flowing, behaving like a Newtonian fluid, the substance is called a "Bingham fluid" (plastic fluid).
Substances that decreases in viscosity when force is applied to them are called pseudoplastic fluids. Before force is applied, these substances have a high viscosity, appearing at a glance to be Bingham fluids, but they do not have a yield value. Some common examples are foods that can be packaged in soft, tube-like containers such as mayonnaise or ketchup, which are almost all pseudoplastic fluids. Another type of fluid that behaves similarly to a pseudoplastic fluid is thixotropic fluid (※1).
Dilatant fluids are the opposite of pseudoplastic fluids. When force is applied to dilatant fluids, the viscosity increases. A typical example of a dilatant fluid is a 1:1 mixture of starch and water. When the mixture is gently poured, it flows like water, but when quickly mixed with a stick, it thickens into a stiff mixture that becomes difficult to pour.
Thixotropic fluids experience a decrease in viscosity when force is applied. In this way, thixotropic fluids seem to be similar to pseudoplastic fluids, but a major difference between the two types of fluid is that the viscosity of a thixotropic fluid changes with the passage of time, as well as when force is applied to the fluid. When constant force is applied to a thixotropic fluid, the viscosity decreases, but if a thixotropic fluid with decreased viscosity is left alone for a certain amount of time, the fluid will return to its original viscosity. For example, if paint is stirred its viscosity decreases, making the paint easier to apply with a brush or roller. The reason for mixing paint before application is not just to make the color more even, it is also to draw out and improve the thixotropic properties of the paint. After paint is applied, it no longer has any force applied to it, so the viscosity increases, allowing the paint to dry without dripping. Perfect paint that sticks to the wal l and does not drip is taking full advantage of thixotropy.