viscosity and friction are two very important concepts in fluid dynamics, fluid statics, solid statics, and many other areas of science. They are also two phenomena we see all around us and they are really easy to understand given the right approach.

Viscosity quantifies the internal frictional force that opposes fluid layers in relative motion. The shearing stress or strain generated by this internal force depends on both the velocity gradient and the shear rate. Viscosity is different from friction because the shearing force in a fluid doesn’t depend on contact surfaces but rather on the layers of fluid that are moving relative to each other.

While it is difficult to measure the viscosity of a fluid, its effects are very important for understanding fluid dynamics. A fluid with a high viscosity feels thicker and moves slower than a fluid with a low viscosity. This is because a higher viscosity fluid requires more energy to be deformed.

While it is hard to define exactly what causes a fluid’s viscosity, it is generally understood that the shearing stress produced by a fluid must be proportional to the shear rate and the velocity gradient above the surface. For a Newtonian fluid, this means that the momentum transport is governed by discrete molecular collisions and that it is essentially the same as what happens in a stationary fluid (see this article). For non-Newtonian fluids, the viscosity is determined by other factors such as temperature and shear rate.