Newtonian fluids maintain a constant viscosity regardless of external stress, while non-Newtonian fluids can change thickness. Examples of Newtonian fluids include water, some oils, and air. Changes in viscosity can occur due to temperature and pressure, but the linear relationship between viscosity and shear stress remains.
A Newtonian fluid is any fluid that exhibits a viscosity that remains constant regardless of any external stresses that are placed upon it, such as mixing or a sudden application of force. An example is water, as it flows the same whether it is left alone or vigorously shaken. This can be counteracted with non-Newtonian fluids, which can get thicker or thinner when stress is applied. It is possible for the viscosity of a Newtonian fluid to change if it is exposed to different temperatures or pressures rather than external force applications. Many fluids become denser when cooled, for example, although they still react to shear forces without changing viscosity.
The defining factor of any Newtonian fluid is that it will flow the same way when a large force is applied as it will when left alone. This means that it can be mixed vigorously without changing its viscosity. Another way to describe these fluids is that they have a linear relationship between viscosity and shear stress. Regardless of the shear stress applied to these fluids, the coefficient of viscosity will not change.
Many different fluids are Newtonian, including water, some oils, and air. The importance of this fact can be seen when looking at the many different fluids that do not share this characteristic. Some fluids exhibit shear thickening, which means that the viscosity increases in the presence of an external force. If water or air exhibited these properties, runners and swimmers would find their surroundings thicken as they try to move faster. This type of effect can be seen when water and cornstarch are mixed to create a non-Newtonian fluid, which is liquid when left alone but solidifies and allows someone to pass through it.
It is possible for the viscosity of a Newtonian fluid to change through factors other than externally applied force, such as temperature and pressure. Compressible liquids will tend to thicken under pressure, while incompressible liquids will show negligible change under the same circumstances. These fluids can also change in density when exposed to extreme temperatures, which can lead to an increase or decrease in viscosity. A fluid that has changed viscosity through any of these methods will still exhibit a linear relationship between viscosity and shear stress.
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