What is viscosity?
Viscosity is commonly referred to as a fluid’s internal friction or resistance to flow. This is a characteristic of liquids and gases that is produced by intermolecular forces. Viscosity has a very high dependence on temperature. As the temperature increases, the viscosity of liquids decreases while that of gases increases. In other words, liquids flow easier in high temperatures while gases flow with more difficulty. At higher temperatures, the molecules in liquid have more energy and can overcome intermolecular forces more easily. They spend less time with other molecules since they have more energy. Gases, on the other hand, bounce off of other gas molecules more frequently. Since there is a higher frequency of collision, it becomes more difficult for gas molecules to move smoothly past each other.
Dynamic and kinematic viscosity
Dynamic viscosity is the quantitative expression of what we were talking about. It is sometimes known as absolute viscosity. Kinematic viscosity is the ratio of dynamic viscosity to the fluid’s density, seen as a ratio of the viscous force to the inertial force. Sometimes the kinematic viscosity is called the diffusivity of momentum since it can be seen as how much the fluid’s motion dissipates as it flows. As a fluid flows, the kinetic energy of the molecules transfers to the environment and to other molecules. This transfer is the dissipation. Kinetic energy and momentum are linked, so as kinetic energy dissipates, so does the momentum.